I would spend some time eliminating the usual suspects in the electrical system
such as bad connections and earths, possibly refit
the original starter. Check the battery with a load tester if possible and if all this stuff checks out ok I suggest the possibility of tight pistons or perhaps rings not gapped properly on re assembly.
How extensive was the rebuild
? Was it done by a reputable workshop? Have you done a seatrial and put some load on the engine?
Some time ago a similar thing happened to a boat
in our marina, a newly rebuilt engine and it simply stopped on the sea trial , was towed back in with a suspected fuel
problem and magically restarted when cool and ran perfectly again until about 75°C then stopped again. The problem was that the machinist had made the fit between the cylinder and the piston too tight and as the piston expanded , the friction increased so much it physically stopped the engine. The solution was to re hone the bores to the achieve the. correct clearance, a tedious job that involved stripping the engine to a bare block again and transporting it to the reboring guys workshop. The pistons were scuffed a bit and could have been reused but the owner decided on new pistons and rings( properly gapped) and the engine lived happily ever after.
How hard is it to rotate the engine using a wrench on the crankshaft pulley nut when the engine is hot compared to cold?
I'm hoping it's just an electrical