what is there now?
easy rule-of-thumb is,
petrol - petrol & diesel - diesel.
also.
4cylinder interchanges easily with 4 cylinder, 5 cylinder with 5 cylinder.
its reasonablly easy to convert from one fuel type to another but swappling fron 5 cylinder to 4 cylinder theres a lot of diferent parts used by vw (like mounts, pipes, sumps etc.)
my '95 t4 was the 1.9d (1X code, 60hp, 4 cylinder) & I converted it to the infamous & popular 1.9tdi (1Z / AHU code, 90hp, 4 cylinder) from a B4 Passat / mk3 Golf
I've done many similar conversions including from the 2.4d / 5 cylinder to the 1.9tdi (4 cylinder) so ave a practical knowledge of such in t4's
I also have a 1.9tdi converted mk2 Golf
Rich.