A VR6 from a MK4 Golf or Jetta will bolt in, but the wiring may have different lengths since it was made for Golf or Jetta. Also, you'd have to deal with the immobilizer stuff... If your bug has a good running 2.5 in it, I would just keep it like that honestly. If you want to do a VR6 build, you should just look for a 98 or 99 with a bad engine. I've seen some go for around $400 where I live.
I have no experience with engine swaps, but this is just what I've learned from reading other threads.
This site, is primarily geared towards; the watercooled Beetles, specifically the 1998-2019 models. You might join and post up on the "samba" site, which is dedicated to the air cooled VW's. Good luck!