While I don't have an Aqua-Hot my Hurricane works on the same principle and I did indeed think about a conversion to propane...it really shouldn't be that difficult to replace the burner, controls, add a regulator and safety circuits. Certainly nothing a few 1,000 bills couldn't take care of.
However, if I only had an 81 lb tank I would be a bit reluctant...not a lot of propane. My 45,000 BTU burner would go thru ~1/2 gal of propane per hour compared to 1/3 gal per hour of diesel so the cost of operation would be close and I carry a lot of diesel. I figure the paypack would be less than 10 years.