I had access to a big metal band saw. My 25 was a short shaft and I drilled a hole in a short 2 X 4 to stick the shaft into. With the shaft down in the hole, I sawed the lower part of the flange from the crankcase.

On the backplate, You examine the part that goes closest to the crankcase. The new rear mount bolts over this existing back plate. You want to remove everything that sticks out beyond the crankcase on the sides. You want to make the back of it flat so the new plate will rest evenly on the engine.

You may have already ruined your flywheel as far as getting Wacker to machine it for you? I broke all the fins from my flywheel and cut them off flush with a air powered Die Grinder and a A11 carbide burr. I then balanced the flywheel using a High Point prop balancer.

There's a little hole above your exhaust outlet that will need to be tapped and plugged with a 10-32 set screw.

Mine has all Carr Precision parts but Wacker parts look pretty much the same.

Enjoy,

Jim