ok first thing thats a m825- Variant with M40 106mm recoilless rifle mounted on rear.
i will guide u to keep every thing genuine as it .
u can replace the engine but if u r replacing the engine replace the gearbox and diffs as well. will guide u to get a 3sefe (that's what in my brothers jeep) it runs great and has ALOT OF POWER.
genuine engine parts are available only major problem with the genuine engine is its economy which is abt 6 to 8km /l but u can increase that by replacing the genuine carburetor and distributor.
if u r restoring the jeep first get the car running and drive it for 2-3 months till its mechanically flawless and then get it dented painted.\
do note a good restoration will cost u b/w 350 to 5lakh .do not think u will spend 1lakh more and it will be fully restored
and yes army green is not allowed