Not easy to buy parts anywhere...  it's a limited production model for sure... 

The SC 500 was a 73 year model and had a silver tank .
For 74 they were designated MX 500 A and had a yellow tank .
I'm pretty sure it was largely cosmetic change , but in your parts searching
using the correct designation might help locate the parts you need .
Back in the day I remember one chap had a street legalised one here in
New Zealand .It used to seize quite frequently , resulting in a day spent
digging the rings out of the smeared piston . I think the parts weren't plentiful
even back then .

Well there ya go. Was not aware of that. Must've been asleep at work that day lol. Thanks Swoop56.

I was working for yamaha when the SC500 came out, they were coming out with a new style piston so in the meantime 
had me drill slots on both sides of the wrist pin.