21mm shouldn't bee too much. Mines still looked like they had plenty of strength at 20mm even after I linished them and had them lightened and balanced.
I'd say it might cost alittle extra to bore out to 78mm but not alot more...maybe another 50% ontop of a normal rebore.
Can't say I've heard of anyone using the skyline pistons yet though.
A few have used the Subaru Justy ones I remember reading on US 1200 site but unsure what rods. This gives 1608cc.

I'd be buying 1 piston...or getting it out of a junked engine and having a look before you commit.
What I did was bought a piston.....and a spare rod......and machined the piston down to fit the bore diameter......mounted it on the rod and dummy fitted everything before I spent the big bucks.
No point buying lots of crap that doesn't fit.

Cheers
Neil