Isn't it something like 93mm? It's not super crucial. What you will need is a step drill though. The bolt holes cut much better with a step drill.
The good news is I have found the template for the snorkel holes.... the bad news is the hole size isn't marked on it, just the hole location.
If I can find the fitting instructions I guess it will say on there, but they are being elusive.
Sorry Mark?
I find that drilling through thin bodywork with a normal drill can distort the metal. The drills tend to grab. I do a pilot hole first and then use the step drill. It cuts a much nicer hole. Fitted a good many snorks now and the step drill has been superior in my view to conventional twist drills.