Hi Evelyn,
N64 can be done, I’d look to our friends at retro upgrades for that – https://www.retroupgrades.co.uk/product/n64-ultrapif/
Megadrive/Genesis the same – https://www.retroupgrades.co.uk/product/sega-md-genesis-region-dfo/
SNES – Unfortunately we’ve had issues getting the parts for the SNES region free for quite some time.
On the SNES and the Megadrive the cartridges are also a different width, so you’d need to dremel widen the cartridge slot or use an adapter. The N64 has a simple plastic bracket that prevents the insertion of different region cartridges.
Some PAL games will work when forced to 60hz (indeed some PAL releases were literally just the NTSC versions on the same ROM with no optimisations for speed difference at all) but many won’t.
If you’re planning on using a scaler rather than a CRT, check before you buy a pre-modded system if it has a DFO type chip installed with the mod. Many of the older mods did not and this can cause issues on more fussy, modern TVs as the refresh rate is ever so slightly out of spec.