Hey thanks for all the replies!
I pulled everything apart and gave it all a good brighten up with some emri paper. It was still cranky and would only start occasionally after I had let go of the key. Once it was going it ran well though. I was contemplating changing the ignition module or distributor as its a converted presolite dizzy with a pertronix ignitor and I had read they can cause dramas.
Before I did that I got a new ignition switch and after a couple of floods it started. Once its been warmed up it has now had many starts with the key and seems to be causing no problems. I'll leave it until tonight and try it again as I'm not keen on having to paddle it home so I want it to prove some reliability on the trailer.
Thanks again for your help. Not many old mastercrafts here so help can be quite thin on the ground. Really appreciate it!