If you have to use a battery then just switch a fused +12v supply through the key and relay all the big drain stuff that needs it.
Most standard road legal pit/monkey's I've seen don't have any relays apart from a starter switch relay and they work fine. The lighting circuit is normally seperate so wont be switched through the ignition.
The custom loom on my monkey uses a non relayed ignition circuit, but the high and low beams are relayed (for a good reason
) as is the horn due to having twin-tone high output horns fitted.