A 1/4 inch diameter toggle will probably fit the hole if you like a round peg in a square hole. I usually prefer DPST or DPDT so that I can switch both incoming AC lines off. That completely disconnects the line from the amp however a SPST switch will work as long as you use it on the "hot" AC line and not the "neutral".
I did some research on the switch issue about 6 months ago. I wanted to make sure that customers got from me a quality power switch that fits that original slotted opening. I have used over 20 of these switches >> the 1623 switch from CW Industries which is carried by Mouser Electronics.
The 1623 switch looks from the outside just like the original switch but the contacts are real heavy duty and are rated at 16 amps @ 125 volts. Overkill ?? - maybe - since the fuse blows at 3 amps anyways. It's a lot nicer than the original power switch that all the ST-70's and Mark II's came with and is rated for 100,000 duty cycles.
Wow, Bob -- looks like a neat option. Thanks. Had I known I may have gone for one of those.
As it is, I did finally just decide to go with an 'original' from Dynakitparts. I'm going to put a 0.02uf cap across it and be done with it. The originals did last for quite a few years so it should be just fine.