Sexton R975-C1 a mess

[carl brown]

Your engine definitely failed due to a worn out crankshaft main bearing, resulting from no oil supply due to incorrect operation as David says.

You can fit a C1 crank in a C4 but not the other way round. The C4 crank isn't machined to take the C1 oil pump drive gear or lock ring.
Con Rods are the same in both engines but master rods different.
Pistons are basically the same but there are different rings depending on chrome or steel cylinders.
I can help you with parts if you can't find anything your side of the pond.
Carl