Your situation is different, but I have taken sleds through the boarder at Haines, AK and Beaver Creek, Yukon Territory many times and have never had customs check or ask about the sleds.
I bought a sled in Glenallen a few years back and brought it through those boarder crossings and it wasn't registered to me and the guy I bought it from didn't have the registration and the tags had lapsed. I did get a bill of sale from him. But it didn't matter, no one asked for any proof of ownership.
I have gone through these boarder crossings many times with sleds that I didn't own and never had a problem. No one ever asked or seemed to care.
But Alaskans travel through BC and YT with sleds all the time, so it may be very different going through the southern boarder crossings. And none of the sleds were purchased in Canada, so your situation is a little different.
I would suggest calling US Customs and asking. I have done so many times and they are usually pretty helpful. However my questions have been about transporting firearms and game meat through Canada, but they were very helpful. I just Googled Haines US Customs and got the phone number and called them. Since you are bringing it into the US, that is the only one that you will have to deal with.
Hope this helps.
