Ask a few salvage yards what they want for an engine that runs preferably from a rear end smash

Bent con rods are probably favourite

If the body is earthed properly to the chassis and battery, I suspect there is a connection that has been crimped onto insulation rather than bare wire in your circuit

Merry Christmas

They don’t like 12v

Clutch plate fitted wrong way, or splines need a little lube so clutch can release

Or if you have a towing socket, it could be moisture ingress

Lock it, or lose it, as the saying goes

Too many red herrings!

Does the broken u bolt look to fit the bracket?

Bracket usually found on plant trailers, to stop overrun brake cables from dragging on the road, I suspect the tyre caught it, and flicked it up under your vehicle

The big end cap looks terribly hammered in the pic, were the bolts loose when you dismantled, or is it a poor photo?

What condition are the main bearings? Are/were both thrust washers in place ?

Wrong thickness head gasket?

Check your top engine mounts for a missing bolt

Did you oil the shells at fitting, and did you prime the pump before assembly? Are the crankshaft journals in tolerance?

Does compression improve with a thimble of oil in the bores? did you deglaze the bores before rebuilding?

They’re busy stealing hi luxe’s round here

If you run through vehicle settings, you probably can, if not you’ll need a plug in tool, gap or the like

Did the new clutch plate slide nicely on the input shaft before you did any assembly?