There's a dead diesel Jetta near me for $300. I bet I can find a non-running Caravan for that or less and rebuild the 1.6 to drop it in. Tough part would be finding a 5-speed Caravan. I had a '89 Voyager with the 2.5 Chrysler four and a 5-speed once, I should have kept it!
Better yet, there's enough room under the hood of that minivan for a tractor diesel. It only takes 48 horsepower to get a 3600lb vehicle (think 90s Grand Caravan empty weight) to 65mph in 35 seconds. I don't even need to be that fast. I would think a 36hp tractor motor (very common power level for garden tractors like Kubota, Mahindra, etc) would be enough to pull a minivan up to highway speeds and keep it there. Might need to play with the gear ratios a bit to wring more oomph out of it but hey, it's a project.
What would be more fun than that? Putting a 6.5L turbodiesel in a Camaro. Sure it's been done, but getting 35mpg out of a diesel Camaro would be...well...it's a diesel Camaro, what more fun could you have?