Just my 2 cents worth....The crankshaft and pistons are not the same. I did not look up the jug. Needless to say, you are looking at a major project and expensive to boot. New crank about $500, plus pistons around $75 each, machine shop work. Then your ECM will have to be replaced....So far the manufacturers are keeping their information secret. Even though computer controls have been around for a while, I could not find an aftermarket code reader for any of my Polaris or motorcycle. My Bike is a harley, but nothing is available for any of the Motorcycle, ATV or Snowmobile manufacturers. So I would doubt you would be able to easily get the 700 ECM reprogrammed. I can see custom tune=$$$$$$$ if you can find someone to take on the project.
As for NOS, I would guess that it is possible, but it would be difficult to accomplish. ATV market would be small for anyone to make a kit, so you would have to source everything and make your own parts to get it to work. Then you still have the problem of tuning. NOS is an oxidizer...adds Oxygen to the intake so you still have to add extra fuel for the engine to burn or it will be lean and melt the pistons. Have to go with a wet plate style or source bigger injectors for the EFI. Same issue with sourcing parts and tuning to make it work. You would also have a high probability of breaking parts. NOS adds a lot of extra stress to the internals of an engine. Unless you went with custom made crank, pistons and rods, there will be failures there. Stock parts are not made for that kind of abuse
Don't get me wrong, anything is possible....in a local auto shop, I saw a Harley sportster with a roots style blower on it....If you have the time, money, expertise and patience you could get it all done.
With that said, I would guess that you would be farther ahead to spend the money on a complete 800 engine with the ECM to make it run. Then add whatever performance parts you can find for it to get what you want out of it. It would make an interesting project though.....grab a handful of throttle and smoke all four tires....
Mine: 2018 Sportsman 850SP - 26" Bear Claw HTRs on OEM steel wheels(I really don't like the Aluminum ones)
Polaris Hand warmers, Mirrors.
Hers: 2019 Sportsman 450. 26" Bear Claw HTRs on Moose 427X Wheels.
2007 Sportsman 500 EFI, Polaris Winch, 25" Bear Claws QSC Clutch Kit, Hand Warmers
2001 Magnum 325 MOSE 12" rear wheel conversion. 25" Sedona Coyote tires
2017 Polaris Ranger 1000XP - Roof and front glass
2018 Kawasaki KLR650
2001 Magnum 325 Warn winch and Moose 60" plow-Sent to Daughter and Son-In-Law since his blew up
SOLD 2004 Sportsman 600 Twin