Recently I found an Ampeg B-100R that could have been the very same I had when playing in bands 2000-2010. Missing back panel showing a burned up circuit board, sold for parts/repair $25. I wanted to keep the original chassis, so one major constraint is the clearance for tubes (red arrow), 3 inches. That's enough for any of the 12A_7s and their like, the 6DL5/EL95 (a sort of half-pint EL84 for car stereos), and -- just barely-- the 6BQ5/EL84.
Laid out some parts. I think the push-push switches on the original are cool and with a little modification these Fender-style slide switches will be a good substitute in the same positions.
Made some mock-ups of the new control panel, with some Ampeg-flavored choices to make about which controls to build (while not copying exactly the B-100R or B-15 or SB-12 or SVT).
Modified the chassis to fit the new parts (full-size pots with 3/8" bushings, slide switches, 11/16" pilot).
The original power transformer fit 2.5" x 2.0" Z-mount cutouts, so a Champ/Princeton sized PT dropped in. I added holes for a fuse holder, Marshall-style capacitor clamps, speaker jack, and three tube sockets with some old parts to check the fit. There's room for one more socket at the far right if need be. The output transformer may hang outside the chassis beneath the cap cans or may go inside the chassis above the speaker jack. Gotta find a scrap of blue checkerboard tolex to replace that missing back panel.