I think smoke/fire anywhere *inside* the aircraft is probably the worst emergency you'll ever have to deal with. I'd agree that the first thing to do with an obviously electrical one in a light aircraft is to turn everything off, removing the initial heat source, at least.
My thoughts then would be for an immediate forced landing/ditching, unless it was obvious the fire was completely out. You have so little time to do anything if the fire re-ignites or spreads out of view. Unless an airport was literally a minute or two away, it'd be field/road/lake/river, etc. Without 100% O2 you can be overcome in seconds - even then, you may not be able to see enough to do much other than crash.
As far as using the radio - well, that might be the source of the fire! Hopefully, someone might see you land and ring in with "I've just seen an aircraft go down in flames!" and be right for once...
Not a nice place to be.