A tractor is considered personal equipment, personal property, not a motorized vehicle for either road way, (plates/tags/reg) nor a watercraft (stickers/reg) nor a motorcycle or ORV (also tagged, if used on State ORV trails.
Thus, it is no different than your lawn mower, chain saw, or snow blower which have none of these things either. Nope, here in Michigan, there is no title, registration, nothing.
Bill of sale. This is as it should be. All the things mentioned above may be of some assistance in terms of law enforcement and legalities, but they are also just another form TAXATION or revenue stream.
My $.02 Have a nice day.