I've never restored a cab, so take my opinion and toss it, for all it's worth
Buuuttttt. I'd say if there's anything "of interest" on the interior (like serial numbers, instructions, specs etc), just clean it and leave it.
But if it's all just mdf and plywood and a bit of overspray. why not give it a once over with some 220 grit and shoot a coat of black over it?
I guess in 50 years, if the cab ends up on Antique Roadshow, having sprayed the interior might render it worthless, but other than that?