https://www.ebay.com/i/173031895561?chn=ps

I'm interested in practicing runs and hopefully doing some eventually, would these be legal to use if I obainted a Japanese sm64 cart?

If you already have an US sm64 cart, you can swap the backs of both cartridges so the Japanese cart can fit into the US n64.

It's alot cheaper to get a screwdriver to swap the backs.

Screwdriver for the n64 security screws: http://www.ebay.com/itm/162210303297