The conversion is pretty straight forward. The NG vehicles used different parts from the intake up. The shortblock should be identical. What I don't know is how NG affects the rest of the engine. Some fuels reek havoc on engines, like Alcohol and mixed fuels. Propane is pretty gentle, but I've not had direct contact with NG. All I can say is maybe do a search on the internet for alternative fuels and see what you turn up.
Otherwise, here's what I'm thinking: Even if the engine blows up into a million pieces on your way home, you could have a new engine installed and still be bucks ahead. A 97 van stripped down with V6 and no A/C specs out to 7000-8000 retail in NADA and KBB. For $5500 (provided its not a salvage title or has a severe history) you have a good deal. For $2000, have someone put a new gas engine in it and drive it for 150k miles with a smile. Sell the NG engine on Ebay and get back some of your money
