First, what brand and model is it? Then see if you can find a copy of the Sams Photofact which will tell you the specs of the power and output transformers. Once you know the specs, you can figure out what other power tubes will work best. You could, of course, just rebuild it using the stock type tubes (7868s?) since it is already a SE amp. If you don't want to deal with the complexity of the tuner section you could rebuild it in a new chassis.
Years ago I pulled a somewhat similar receiver from a Wards Airline console that I found at a flea market. The amp section was SE 7868s. I used the output transformers to build a budget version of the Bugle 45 SET. Power transformer came from an old Philco radio, I think. Not a lot of power but it's a very nice amp.