The flywheel I used is from Kennedy Engineered Products (no relation).   It
cost $290 and is a beautiful piece of metal.  For a smaller amount of money
they will put the appropriate ring of teeth on a Porsche flywheel you provide.
Transatlantic shipping wouldn't be cheap, but I would still recommend
buying vs. making this part unless you have a wonderful machine shop handy.
The KEP flywheels are made to order, not stocked.  You might consider asking
to have yours made a little thinner:  On both engines I have mounted my
flywheel on (71 911T and now 77 911S) I have had to machine about 1/8 in.
off the boss for one of the crankcase bolts.
Email me direct if you have other questions.  And good luck -- 125hp with
reduction gearboxes sounds like something that could lift the front wheels
on a good day.
B.