Here's a simple way to get those screws out in future.

- Use the screwdriver/sparkplug spanner that came with the engine
- Place the engine firmly between your legs with the plate facing upwards
- Put the screwdriver into one of the screw slots & give it a good WACK with a hammer - this cracks the thread lock & seats the screwdriver into the screw.
- Unscew the screw from the back plate.

I used to have all sorts of issues getting those plates off until I was told this trick by an engine modder. It has wroked every time since.

If you use a dremmel & scut off wheel as Ron said you still may be able to use this trick.

Cheers
Jason