If you’ve got an NTSC Nintendo 64 then it’s very easy to modify it to play both American and Japanese N64 games. Open up the cartridge slot doors and look for the small rectangular cartridge tabs, all you need to do is remove them, on US consoles they are in the corner, on JPN consoles they are a bit further in. Unfortunately this will not allow it to play PAL games though.
Ideally you should take your console apart using a Gamebit, some people use a soldering iron to melt the tabs away without taking the console apart – I don’t trust myself to try that method!.
Once you’ve removed all the Gamebit screws, remove the Jumper/Expansion Pak, then remove the upper half of the case. Undo the 2 screws holding the cartridge door assembly to the case and proceed to remove the tabs.
Now you could cut away at it with a strong sharp fine craft knife, you could melt it away with a soldering iron, you could drill them out or even saw them away using a junior hacksaw. Whatever method you use, tidy the job up by filing the rough edges away.
The following photo shows the missing cartridge tabs missing from an American N64.
I can’t remember where this information was found, but it’s widely documented on the web.