We'll I'll go again on topic. I reread everything and the new engine does NOT have a counterweight inside...correct?(this is like an opposing camshaft if I recall). So that might tell me it was designed for a much larger flywheel to correct this or it is not made to run at low rpms like a tractor is (say idling or throttle changes,
FEL work, etc).
When I worked on engines 1 engine model could have several different specs. This could different pump specs, block drilled for external pump, etc.
But ours did not have an internal counterbalance shaft. About all you can do is give it a go.
Or, and I hate to say this, fix the old crank and install it in the new engine.
Let us know how this comes out please.
Good Luck,
Rob