I just think hanging a prop on a crank directly is a non-starter in
the first place...especially on a crank and case not specifically
designed for this in the first palce. Maybe a good redrive and flywheel
would be a better way to go?

That is my opinion, also.
--
I partially agree.

I agree that hanging a prop directly on the end of an automotive crank, even
if you put it on the flywheel end, is a recipe for a short tbo even at low
power and for a high failure rate at high power. (If you only use it to
push an airboat around the local swamp, you can keep a couple of bottles of
skin-so-soft in your tackle box.)

However, switching to a traditional aircraft powerplant may not solve the
problem. You really only have the full value of testing, experience, and
service history when you mate an unmodified engine to an airworthy propeller
with which that engine was certified--and preferrably in a combination used
by a large number of aircraft in regular service for a reasonably long time.
Remember the crankshaft problems in some of the Cessna 172's soon after the
change from the Continental O-300 to the Lycoming O-320. (I think it was an
early 160HP version, but have long forgotter the dash number--and the
problem was promptly solved.) There have been other "teething" problems as
well on various engines...

In the special case of a KR-2, which was the subject of at lease two of the
Corvair engines torn down and inspected, the plane sits too low to swing a
70+ inch diameter propeller; and IIRC was originally designed for VW engines
swnging 52 inch diameter propellers. I have heard that the KR-2S sits
enough higher to accept a larger prop, possibly 60 inch diameter. That
seems to negate the reduction drives, although a shaft drive, similar to the
one Steve Whittman developed for his V8 powered Tailwind, could be
interesting. BTW, the plans are still available--I think Aircraft Spruce
still sells them. Also, Revmaster (and possibly others) offers an aircraft
engine based loosly on the VW dimensions and a Jabiru could work--especially
with a 3 blade prop...