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Fuel for the fire: if I built a custom frame, I'd have to register the bike as an experimental or something? Just curious.

Sometimes, it is cheaper, (depending on your local Zoning/Tax area)
to, 're-model', a house, than build a new one.

Could that be the same with a MC. That part, that has the vin, (that one has a clear title for), is retained, and then the bike/frame, is, 're-modeled'.

Don't know, check where You live, if you want to do that.

In Minn, as long as the vin number, AND, the motor, number, match.
Just write a check and get new tabs, or transfer.
(And on a title, the required space for the motor number is, BLANK, so when you transfer, YOU must fill in the motor number, and, 'they', will tell you if it is correct) If not, no transfer or license.  If you have a title to a frame, and put on a new motor, Then, you have to have it, 'inspected', for the new motor number. (As well as the rest of the nit picking)

In places where one has to have a yearly inspection.
Well, if all things, 'inspect ok'.