I know there are tube amp guys that do this kind of work as a side project.
here's an example of a serious-minded effort
I just googled "converting old PA head to guitar amp" and got 6 or so links
At least, I contacted one myself when thinking of converting a couple of similar pieces I have (stromberg-carlson and RCA). But I never sent it away as the items are heavy and I wasn't that committed to the project. I think the dude wanted 2-300 dollars for the work. It's interesting and I'd like to know what you find out. Sorry I couldn't be more help