Unofficially, one of the taxi drivers outside the gates can sort you out with an Enfield for about 300-350 rps per day and the blue house next door sells petrol by the litre bottle...
Officially, remember the rental laws are being more tightly applied and you should have a proper rental bike, helmet and documentation if you're stopped. There's a place in Panjim that does it all legally.
Never bothered before this year but i saw a kid get mashed into a tree while flying past me and it shook me up somewhat. Dangerous riders everywhere.