Ok I don't have access to the regs anymore but I did work customs up there for many years. People typically bring a tractor back themselves on a trailer. You need NO CONTACT with Canada Customs whatsoever beyond telling them what you are doing going up there. Typically you DON"T need a customs broker and the advantage and cost / time / headache savings is all yours just taking your own truck and trailer. Now this is for a PERSONAL IMPORTATION. That means to use around the yard and back 40. DON'T take a truck with "Bobs excavating" on the side or they are going to spin you right over to Commercial Cargo to find a broker. You just want to be a couple guys who went north to buy a tractor. They will probably direct you there anyways so they can see it before they cut you loose.
You need a bill of sale showing the serial number model ect like you would a car. There is NO customs documentation that they will give you and none are needed as you aren't registering it. You will have to pay a pittance of duty as it's a Japanese made tractor. All duties are figured by the COUNTRY OF ORIGIN not where you are bringing it from. You just take it home and use it.
You have just ONE POSSIBLE WRINKLE IN THIS AND IT'S A BIG ONE>>>>>>KUBOTA................This is why I avoided them in the first place way back when. Somebody somehow got KUBOTA stuck in a special classification in the Tarriff Code Of The USA. I swear it must have been the company as it makes no sense from the governments interests but here it is. YOU CAN'T BRING KUBOTA ANYTHING INTO THE USA UNLESS YOU ARE A DEALER. That's why I told them to buzz off back in the 90's as the buck was up and anything I would have gotten would have come from up in CA and they gave you the CA sales tax back including on a whole tractor itself. It appears some sort of protectionist thing that Kubota USA may have gotten put into law to keep dealers happy along the border areas. Stranger things have happened but it is a kick in the crotch for you most likely. Remember, this is ONLY AFFECTING KUBOTA and no other make/ model.
Sorry to rain on your parade but this is something that NOOOOBODY knows about including 8 out of 10 CBP officers but if you get sent over to commercial cargo and some rookie is clueless and decides to check then you would be stuck taking it back. Now it may not really be a deal breaker after all . It may only mean that you have to use a Customs Broker(thats US side again no Canadian involvement officially). They would do the paperwork for you and of course you give them around $200 - 250 IIRC something to that effect but that's IF they can even do it for you at all.
First thing to do is see if the TUSA (US Tarriff schedule is published online on the CBP website), it probably is somewhere, and see if what I just said is still in effect. I last looked at this around 2004 so it could have changed. It may even be published elsewhere on the net as well. Just make sure before you set off on the adventure.
If you end up going remember it's Quebec............land of words you can't even pronounce so take a GPS that has Canada coverage as you will surely have to deal with the St Lawrence River and bridges are like cops, you never can find one. Traffice issues, road closures in Montreal are legendary (at and near those bridges) . Wash all the dirt off said tractor. Not squeaky clean but if you bring back 25lbs of Canadian mud all over the place they will make you go back to Canada to wash it off and you have to figure out where. Bring a Passport. An Enhanced Drivers License works as well but you can usually only get those from DMV's in border states. No passport or enhanced license you don't get into Canada. If you need and can do it with a customs broker do everything in advance and have it ready so there are no surprises. You might not get to do all the specifics in advance but at least have it all set up in advance for when you show up in their office. ALSO ONLY DURING BUSINESS HOURS. Most brokers have shut their satellite offices down after 4pm and do things remotely via computer link. Same with weekends. This saves them big bux on personnel but is geared towards established accounts and you don't want to go there trust me. Probably better to come down I-15 from Canada crossing at Champlain Border Crossing on I 87 too. It's a much bigger crossing with more broker offices ect. and they are all open until midnight. I 87 has less hills to deal with and probably an easier way home for you anyways.
Sorry about the long response but I figured it might help anyone wanting to work a similar deal. Start digging now that you have your homework. Perhaps that greedy Kubota regulation is gone now. Figure that one out first then if it is see how much and if a broker is able to do it. Oh yes, if you need a broker do use one with a physical office at Champlain or the port you choose. And if you do use a broker do take the truck lane just before you get to the border while still on the Canadian side as you have to then go to Cargo to clear. Much of this is likely to be found on the commercial section of the CBP.GOV website though you may have to dig. Good luck.