THERE IS AN EXAMPLE
Lets say l had a dt 100, the keep is legal, l could make that bike look like a yz 100 A, l did own one about 10 years ago, great bike, and it is still around.
Lets say l owned a xr 75, so l wanted to put that engine into a larger frame, with a cut off date of 1977 or even 1976, l have a larger choice, then that 75 4 stroke engine l could make it bigger, or use a 125 engine, that is your choice., at the end of my restoration that bike could look like a smaller version of these honda cheney bikes that are racing in the bigbore races. do you get my drift