Your MPU is not working.

The flash pattern is explained here:
http://www.pinwiki.com/wiki/index.php?title=Bally/Stern#Bally_.2F_Stern_MPU_Board_LED_Flash_Sequence

No doubt battery leakage has damaged the board.

You have to unsolder affected components and clean the board.

Directions here (it's a Williams board, but you'll get it):

https://pinside.com/pinball/forum/topic/vids-guide-to-bulletproofing-williams-system-6

Here is Bally info:

https://pinside.com/pinball/forum/topic/vids-guide-quick-bally-driver-board-repair-bulletproofing

It depends. Are you comfortable working on electronics? Do you have a multimeter?

Start here:
http://www.pinwiki.com/wiki/index.php?title=Bally/Stern

If you are in a hurry and have the dough:
http://www.allteksystems.com

I've gone both routes with repairs. Some of it depends on skill set and time. Some of it depends how far the corrosion has spread.

Whichever way you go, the first thing to do is remove that battery from the board

I would suggest replacing the MPU with this aftermarket one and resell the old one on Ebay.
http://www.marcospecialties.com/pinball-parts/ULTIMATE

If it has been in his garage there may also be some major issues with the playfield. Freezing and thawing in an unheated garage causes the plywood to crack, warp, etc.

FG is a kick a** game I hope you get it working. You are now in this club if you wish to lurk around.
https://pinside.com/pinball/forum/topic/flash-gordon-saviors-of-the-universe-club

yeah also look for rodent damage.

Remove the glass and lift the playfield from the front like the hood of your car.

On the coin door is a latch on the right side

Move it, then remove the long silver lockdown bar at the front. ( it lifts straight up)

The glass now slides out the front, and then the playfield lifts up.

There should be a bar on the right side of the cabinet that will keep the playfield up.

And it's hard to tell, but it looks like the battery has already been moved off the MPU board

The battery has been replaced but there is still a lot of discoloration from the original battery leaking. If it was not neutralized it will continue to corrode. It is hard to tell how much damage has been done of if it was properly repaired.

It appears the battery mentioned that should be removed has already been done. I assume that black and white "package resting just beneath the upper left board is a battery pack. Hard to tell if there is corrosion on that board or not...(Upper left is the main brain...MPU) Can you get us a closeup photo of that board? The lower portion of it is the most important to see.

Also on that board in the middle toward the bottom is a little green LED. When you turn the machine on what does it do?

If the battery was changed and any corrosion was cleaned up, there is no need to replace it.

There are advantages to the Altek board. Better header pins, free play settings, etc. And no worry about battery leakage again. The down side is that it is $200.

There are major issues with that board. It would be an advanced repair. The corrosion goes clear up the right and side and a ton of the ground traces are gone from the lower middle.

Looking closer, I am betting you will need to replace the connectors on both sides of the board too. The corrosion goes up the pins and in to the connectors most of the time.

If you are not getting the correct flashes of the LED on the MPU, check all of the voltages. If they are all good, replace the MPU with an Altek after replacing the connectors.

http://www.marcospecialties.com/pinball-parts/CKBLY-MPU

You will not need the header pins with the Altek board but the kit still makes it easier.

A good crimp tool is a must as well.

Yes but if it has eaten all of the copper traces those need to be replaces as well as any damaged components.

It is also not 100%. It will slow the return of the corrosion but it will come back. A permanent solution is bead blasting or other abrasive removal of all the corroded material.

+1 on the connectors. It's a Bally & it's an '81.. gonna be some issues there for sure.
Good info here
http://www.pinrepair.com/connect/
Tools & repair parts here
https://www.greatplainselectronics.com/categories.asp?cat=35
https://www.greatplainselectronics.com/products.asp?cat=138

.1 & .156 Molex connectors used
Don't get the cheapest crimper, the 1026-CT Sargent is probably the best for the $$

Also - here's a link that has the manual & schematics..
http://www.ipdb.org/machine.cgi?id=874

Check your mail on here, I am also sending you a good repair guide.

So you have a flicker (normal) plus 6 flashes? You're almost there. The 7th flash is looking for solenoid voltage (43V) which I believe comes from the connector J4, that's the lower left connector on the mpu. Try removing and re-seating the connectors on the mpu. sometimes this helps make contact when games sit for an extened preiod. There's a key on each connector so becareful not to bend any pins. No 7th flash can also mean a bad PIA (U 10 and/or U11 ) on the mpu. Go to pinwiki.com and click Bally/Stern classic, then 1977-85 and scroll down to section 4.5.4.1 and it explains the flashes in detail..

I would unplug the connectors and remove the MPU board from the backbox. There are some obviously destroyed traces on the board. If they are repaired with jumper wires on the back of the board your problems may be much smaller then it looks from the front. Post a picture of the back of the board for us please.

Have you been able to make any progress with this project?