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Google ORC section 4503.181
This is the Ohio Revised Code section on historical vehicles. I know of no restrictions on mods or upgrades.
A couple of key points though:
“(A) As used in this section, "historical motor vehicle" means any motor vehicle that is more than twenty-five years old and that is owned solely as a collector's item and for participation in club activities, exhibitions, tours, parades, and similar uses, but in no event is used for general transportation.”
“The owner of an historical motor vehicle that was manufactured for military purposes and that is registered under this section may display the assigned registration number of the vehicle by painting the number on the front and rear of the vehicle. The number shall be painted, in accordance with the size and style specifications established for numerals and letters shown on license plates in section 4503.22 of the Revised Code, in a color that contrasts clearly with the color of the vehicle, and shall be legible and visible at all times. Upon application for registration under this section and payment of the license fee prescribed in division (B) of this section, the owner of such an historical motor vehicle shall be issued historical vehicle license plates. The registration certificate and at least one such license plate shall be kept in the vehicle at all times the vehicle is operated on the public roads and highways in this state. “
