Here in the UK (where NEMA connectors of any type aren't really used at all) I've seen Tek scopes that have been retroftted with EP4 power inlets. Coming from a broadcasting background I have plenty of these around though they are still available new, albeit specified for loudspeaker connection, not mains.
Nice, solid, rugged connectors. They look good too, especially the older types. It doesn't help that there are 2 different wiring conventions which has caused endless grief to many broadcasting engineers. In the USA, I don't know if they are found at all. Vintage Marconi broadcast studio kit may well have used them, even in the USA.
Another approach is less good looking but inexpensive and effective. The very common IEC C13/C14 mains inlets can be fitted by using an adaptor plate. I don't think any drilling of the original would be needed though would need to check exact dimensions.
ppppenguin