09 Polaris switchback 800
I believe BCDan covered as much as could be possible above wrt a swap in cc.
Imho, repair the 800. That should be your lowest cost option.
Also, you may may not think you need it. However, I can confidently assure you that you will be missing the grunt and thrust of the 800, when your friends are stopped sipping their beverage while waiting for you to come along over that last 15 mile stretch of trail or field, when you are digging put track and pulling on skis instead of having just cracked the throttle, when you are humming and singing along instead of clenched buttocks, white knuckles, and gritting teeth ...
Fix the 8. Or at least start with the disassembly of the engine so you know what you have to deal with. Then make a decision. If you want to downgrade just fix then sell it and buy something else. Or if repairs are too high rip it all to pieces and start the listings of parting out sales.
Those are the options as I see them.