You ask a bit of a loaded question. Traffic laws up here and I believe in the US as well, are regulated by the provinces/states so I don't think you are going to get a blanket answer. For example importing a trike from the US into Canada can't be done unless it is a factory HD trike, as it has to be a manufactured product, but you can import a sidecar rig because it is classed as an accessory not a modification, which the non HD trikes get caught up in. This is under Federal legislation not provincial/state. I'm sure you have similar import/export conditions on items as well. With respect to provincial legislation in Alberta, where I live, if you bring the bike in from another province in Canada or the US ( in addition to meeting Federal Import requirements if the bike is from the US), you have to have the bike safety inspected, tires, brakes, lights, they take it for a short ride to make sure everything is OK, and then you get a piece of paper that you take to your where ever you register your bike. Saskatchewan where I lived before, didn't have safety inspections, just the requirement to run the serial number on bikes with over 650cc ( To see if stolen ). So this varies from province to province and I'm sure state to state. Ask yourself is what you are doing a modification or an accessory. If what you are doing easily removed ( relatively speaking ), then it is an accessory. Accessory's are not going to trigger any real problems as they didn't change the bike itself. Sidecars are a case in point, as most subframes bolt/clamp to the bikes existing frame. EML builds ( at least they did, not so sure anymore ) their own frame and puts the bikes engine etc into it. They are imported into the US similar to any imported car etc, so they would have to meet whatever Federal standard you have in the US. After that I don't think the states are a problem. So if you are really worried about the insurance aspect, then I'd stay away from the mod, but if you don't have any problems sleeping at night, then go ahead and do what you will. What you are doing is really no different than lengthing/reinforcing swing arms and so on. Once its painted, nobody will be any the wiser. Above is just a ramble and an opinion, so good luck.