A camping forum for VW bus and camper owners.
The correct motor for a 1971 bus is going to be a 1600cc dual-port engine. That's a Beetle engine so quality will run the gamut. In general, the more you pay the better an engine you'll get. I would personally stay away from big name budget builders, the ones with two-page spreads in Hot VWs, and would go with someone as local as possible.
Some things to think about:
People will try to talk you into going bigger with an engine (and lord knows pre-1972 buses are underpowered), but any significantly larger engine you'd get will be a compromise with reliability. Stock is best, unless you have loads of money and can convert to Type IV power. Those with 1776cc or 1850cc motors in buses who say that I'm full of b.s. can report back after their engine's given them 85,000 trouble-free miles.
Any builder who wants to set you up with a 009 distributor should be avoided.
Any builder who wants to set you up with dual carbs of any kind (and likely a bigger engine to put them on) should probably be avoided as well, unless s/he reeeeeeeeeeally know what s/he's doing.