Restoring original water heater or cheap alternative?

As you probably know, Atwood currently makes a replacement which fits the same slot as the old Bowen heaters fit. They build them from the basic manual-lit pilot model to a combo unit that will run either propane or electric (which in most cases would also require some extensive rewiring of the trailer to accommodate the draw). There are also models in between which are propane-fired but have electronic ignitions, which require a separate 12-volt wiring unit to operate the ignition. Prices vary and they are not cheap, but are well worth it if you consider the price as investment. Pricing can vary greatly, and you can save a few bucks shopping around. Camping/RV places will be pretty expensive compared to other sources.

After all that being said, fixing up an old one can be risky business, as they can fail at any time, and usually at the worst time,  just like any other old appliance. I have seen them fire up and quit, fire the pilot but not the burner, fire the burner but not the pilot, etc. etc. But as long as you are prepared for the worst there is no real reason not to try.
Two basic things to check. First, the tank will have to hold under pressure. Put the water to it and see if it holds with no leaks (referring to the tank, not necessarily the plumbing leading to and from it) before going any further with the propane end. The tank can be flushed out, using the drain plug to do so. The instructions with the new one states that the tank will hold a couple quarts of water in the bottom when allowed to drain, so running water through will help flush the sediment.
Replacing the thermocouple should not be necessary if the pilot functions properly. They either work or they don't. Modern replacements are a bit different than the originals, and can present problems in replacing. I would steer away if it is possible.

The regulator is another piece of the puzzle that is on or off, and if it is original to an old trailer I would consider replacing it anyway, along with the pigtails for the tanks. The entire system should be checked for leaks religiously, and should all be checked before putting the propane to the system to test the water heater....it is a good excuse to do so, and then the rest of the appliances can also be run safely as well.

I did mine the last thing before it was time to winter-up, and decided on replacement because of some of these factors.

Attwood manual model:



If and when you do replace it, and maybe even if you don't, you might consider a bypass kit in the plumbing.  It makes winterizing much simpler.