What stripped? The head or the threads? Is the head allen wrench or hex head?

Make sure that your alan wrench is in solid or it will strip. I also use a nut driver for leverage on sterns from factory tight. I learned the hard way too

Rubber band or something thinner like latex in head and attempt to turn, Screw extractor, sometimes you can make a notch with a dremel or just drill it out.

If it's any consolation, Bally early SS flipper set screws would crack or strip often. Lots of fun drilling them out in crowded bowling alleys, bars and 7-11's.....

This works great if you have the space to get a Dremel in there. Use the cut off wheel and make a notch across the head where you can then use a flathead screwdriver to remove. I’ve had to do this a couple of times in different situations and it always works out well.

The part is unidirectional, you may have it installed upside down.

We are talking about the screw that grabs the flipper right? on the rectangular metal bar. It’s a crank arm block.

When you insert the screw, the side facing you where the screw enters does not have threads. The other side has threads.

if it is backwards it will never tighten around the flipper and you strip the Allen head.

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For replacements I got black alloy steel screws at a local Ace Hardware.

You could also go fancy and look into the Precision Pinball keyed shaft / crank bar setup.

If you can get a drill in there, use a left handed bit. Sometimes that will back out a stripped screw.... or drill then use an easyout.