Quite a few posibillities
Lack of fuel
Low battery charge , engine not turning over fast enough
Faulty starter motor
Faulty engine earth
Air lock in fuel system
Clogged fuel filter
Immoboliser...
Yes, but you will need a few donor parts of the T25 diesel engine; sump, gearbox bellhousing, outlet manifold and engine support bars. Search for "club80-90" or "brickyard vw" for more help