It will work & as noted you'll need a 3mm spacer plate.
That is an old school big bore build, they did for years until the big bore cylinders came out.
The next size piston in the 85/86 TRX style after the 69.00mm, is a 69.50mm & is the last size you can get in that style, so measure the cylinder bore good to see if theres enough meat left that you can get a good clear bore & hone out of it.
If not, then all you can do is use a 70.00mm YZ250 Yamaha piston & a 2.5mm spacer plate.
You'll need to check the cylinder porting if the YZ piston is used, to make sure the port windows not widened to much, because if they are, the end of the ring could get hung in a port opening because on the ring pin locators in the YZ piston.
Neil
