You should be able to use any tank you want, that would be the least of your problems. Problems you typically run into are mating the different components to each other - different sized ports for your heat exchanger for one, and silencing the blower for another. It is fairly easy to build a frame, and connect a pump and blower to an engine for a simple TM, add an external heater, and you are in business. I have built 2 this way. It took a lot longer than than I expected, and finding mating parts was an issue. It was also pretty expensive. Aligning the different components to function reliably and properly is also a task in itself.