New engine numbers
I ain't a lawyer.
Post 1970 (I think), titles go by the frame number and not the engine number. As long as you have a title/registration for the frame and the motor number is not from another registered bike, you _should_ be OK. For example, your fatboy has a VIN on the frame and stamped on the motor of 1HDxxx. You pull the motor and replace it with a S&S Monster Motor that has a number stamped on it of something else. You have a title to 1HDxx1. You should be OK. You put a HD motor in it from another bike and it is stamped 1HDxx2. You now have something the thugs can start poking around on, running both numbers and seeing what comes up. While you are probably technically still legal, that doesn't mean that they won't give you trouble. Especially if a title exists on 1HDxx2, or if at some point in its life, 1HDxx2 was ever stolen (even if recovered), in which case you are just asking for trouble.
Also, depending on what state you live in, there may be a mechanism to change the title to reflect the new engine number. This may or may not be required to stay 100% legal.
The thugs with guns in Sturgis are terrible on these types of things. They literally walk up and down the streets looking for things like aftermarket cases in an otherwise stock looking bike. They will run acid washes trying to pull up other numbers, know where the VINs are on all the parts, etc. It is really surprising how many stolen bikes they recover, which is a good thing, but they also catch a lot of legitimate people that just didn't have all their paperwork in 100% order, or even those that did. Two years ago, a German couple at our campground had one of their bikes confiscated. It was a theft recovery vehicle, but they had all the paperwork and it was all in order. After about 3 or 4 days of trouble, they gave them the bike back and told them to never bring it to Sturgis again.








