Piece of cake really, if you have access to a small lathe. The flywheel and collet are for the O.S.21VZ-M marine engine.
Cutting the unused part of the SG crank can be done with a fiber reinforced cutting wheel fitted to a Demel,, I've cut
a bunch of them.
I cut the O.S. flywheel "collet" at the front end,, its so long that it makes it get stuck in the flywheel the way it comes.
The flexshaft adapter was bought from Rocket City, its made for the O.S.21.

The adapter (once the nut and collet are removed) is chucked in the lathe and the rear end is turned down to where
theres a threaded part that goes back into the flywheel and grabs some more threads. The part is a stainless steel
model and it cuts like butter in the lathe.
The aircooled head fins normally have to be reduced for boat use. This also is lathe work and depending on your
application of your engine will determine how much fin should be removed to get the heat up.