Are we talking a gas heater or oil? Can you upload some pics if oil is indeed what you mean because that is something that I've never seen.
Hot tub companies typically won't touch non-electric heating units because they are not licensed to do so and aren't going to risk a lawsuit if your unit explodes. Plumbers are flaky at best and if the work is unfamiliar to them it's easy to see them turning down the job. If you've exhausted all of your available plumbers, try talking to a boiler repair or HVAC place. Go in person, bring photos and any info on the heater itself with you and remember that if they say they can't help it's because they don't want your death on their hands.
Depending on age, condition and setup, you may want to rip out the old heater (and possibly most of the electronics) and replace it with an electric unit to avoid this problem in the future. This would require a plumber for the tear out and a spa tech for the conversion.