your single pipe will give you the best torqe, i would just get a good clutch kit, it wont hurt top speed. if you are looking for more top speed, gear up, this will be the best way to get it. even with a good kit you can gear up without losing power.
I have never seen it done, but what says you cannot port a fan cooled engine? engine temp would be the biggest prob, but i am pretty sure it wouldn't kill the engine. In the 70's(haha i wasn't even born yet), they ran a lot of 800 triple fan cooled(or open air cooled) engine, and those thing were prolly ported, so i cannot see why you cannot port that 440 fan. it still won't even be close to a liquid sled though.
slp single pipe, it is loud, but not overberring. get trail porting done and this single pipe and you wont believe the boost in power. if you have a prox, you already have an execellent start. GOOD LUCK.
I think the place that you should try is probably Aaen. They could probably build you a set of pipes, and they have a large Polaris knowledge. As far as porting goes, you would be fine with it. You just can't run it quite so lean in order to keep everything cool.
Personally, Allot of used XC motors for sale on places like Amsnow. A 700 VES ProXC would be awesome with 440F stickers!!
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