Its safe to get dental work done while pg. It is however recommended that x-rays be avoided if possible and that amalgam fillings (silver ones) not be removed or placed.

Unfortunately nitrous oxide (happy gas) is a no no in pg. While it is ok during labour exposure to it during pg has been linked to miscarriage.

Its really important that you get your tooth attended to. As I'm sure you know when you have a large cavity you run the risk of an infection. Dental infections can be very serious and can quite easily spread - especially when your immune system is suppressed as it is in pg.

Sorry for the bad news about the happy gas. Happy gas doesn't have a huge effect anyway - it decreases anxiety but it actually has no effect on pain. Local anaesthetic is what stops you feeling anything and it is totally fine to have during pg. I'm sure with plenty of local you'll be ok.