I would think that the rules should contain:
No mods to the body, must appear stock. AKA restored. factory options OK.
Chassis swaps only for better detail. Revell 69 Camaro chassis under the MPC 69 Firebird.
Must have engine cover. AKA hood, may use factory hood scoops at original height.
May use any engine/ transmission combination subject to perceived horsepower, IE twin turbo LS in a Vega.
Restomod is a vintage body with modern upgrades for comfort, reliability, handling. Not look like a racecar, no roll cages. Maybe subtle roll bar OK.
Tim