Rufus,
Hello and welcome to the forum.
There must be a scrap Lister out there somewhere.
Just need time for one to come up. Left outside,... head off,.... seized solid ....after the big end failed and it was thought not worth recovering. Or someone has made one good engine from two bad ones....
They are probably relatively plentiful in the UK, but it is that shipping cost that kills it all the time.
Seems like your enemy is time, and probably that law which says when you have bought expensive, one will show up for free, straight away afterwards.
What is your time scale?
Scour through all the collectors and magazines/web sites and ask, ask, ask. Look at engine sales and hope there is a 'scrap' engine going for little over melting value.
If you have electric flywheels and find one with the same, those will be of considerable value to someone on this site, for sure (given location).
Good luck in your hunt.
Regards, RAB