70-80k for a kabuli 2C engine.
5-8k for a fabricated sump and oil pump mods.
6-10k for the fabricator to fit the engine in the bay.
5-10k for the gearbox-engine adapter plate.
3-6k for misc items, such as clutch pilot bearings, input shaft bushe etc.
These are just basic known items.... there are plenty of blind spots which would require cash.
A CNG kit might cost you around 40k maximum....
Your call. Its just my opinion.