Um, not sure if your engine was a gasoline one or diesel. If gasoline, put this in another forum.
As far as I can tell, you want to run vapor.
Thing is, unlike running a gasoline engine, you're only adding a /little/ propane to your diesel. Add too much and you get pinging, so you have to keep the ratio low.
If you've got a vapor valve(the fill valve might work?), that's best.
Alternately, if you find one, a small vaporizer unit like they use on a forklift/car would do the trick.
P.S, two things:
1. It's possible to make an IDI /run/ entirely on propane vapor... though very poorly, due to no timing control(it burns when you manage to get a good ratio and heat).
2. You need to have enough air to burn the propane; you'll only get extra power out of the propane if you've got a lean burn already. This means a turbo, or bigger turbo.