Hi groobo
I would first concentrate on "power-on and immediately the Score Motor starts turning".
Lets look at the schema in ipdb.org: http://www.ipdb.org/machine.cgi?id=2803 -> schema.

The symbol for the Score Motor is at 10-E / 10-F. A bit below there is a "fork-off to the left". There are switches drawn "on the left" - and there are switches drawn "on the right".
When one or more switch is "faulty-closed": The Score Motor will start to run.

First step: Look for Reset Relay, Coin Relay, 2nd Coin Chute Relay, 3rd Coin Chute Relay. If You power-on: Does one of these Relays start to pull (this would be the fault) ?

Second step: in the schema (near the "Switch on 3rd Coin Chute Relay") at schema-9-C there is the "Score-Motor-Switch-1-F". It is a "Make and Brake Switch", drawn "closed on the right hand side". It is the "Self Hold Switch on the Score Motor", needed for "once the Score-Motor gets an impulse: "Make a turn" -> this switch makes the Score Motor to make "180 degree turn, complete"".
The middle blade of this switch has a wire "White" soldered-on. The Outside blades have soldered-on: Wire-Black-White /// Wire-Red-Yellow. If the blades are bent and there is always (faulty) contact "Red-Yellow <-> White": The fault.

Third step: Have a look at the relays "on the righthand-side": Outhole Relay, Right Lane Relay, Hole Relay, Center Target Relay, 5000 Relay, 500 Relay, Bonus Score Relay: Is one of these Relays "faulty-pulling" at power-on ?

If You do not find the fault: The next step would be - "considering the Relays are good (not pulling) - BUT a Switch on one of these Relays is bent and makes "faulty contact when relay is not pulling"": You must have another look at all these relays looking for a "Switch faulty-(always)-closed".
Greetings Rolf

Since the kickout hole keeps firing in "game over" mode and during a game you get 3000 points; the kickout hole is firing either due to the kickout hole relay being "stuck" or the contacts for the kickout hole being stuck, shorted, wires to it shorted etc. Time to troubleshoot by looking very close at those areas. Also note a screw that fell off of something on the playfield can do wonders depending on where it lands below.