if u bought engine kosong,u still can register with jpj.but u must get the surat kastam.my car engine also i got from engin kosong/engine lantai.i dont need halfcut bcos i want to change the parts,i like rojak2 ie bigger turbine,bigger throttle body,bigger downpipe,and...ahahaha!!!! secret la.but actually it will cost alot more than getting the half cut.but if u want to heavily modded your engine and got more money,go and get engine kosong.
p/s:my engine also come from engine lantai.still i can manage to reg with the jpj.you just need kastam ltt.