These things are pretty much bullet proof. If it's not completely locked up anyone can do repairs. Parts are everywhere. You don't define what it's doing when you state that it's rough at idle, small outboards idle like an old tractor just normally.
BIG +1 on the Sea Foam fuel additive. It's a yearly ceremony for me along with a new impellor and lower unit lube. I'd do this before anything else. You'll be amazed at the gunk it will clean out of a motor. Practically a rebuild in every can, not bad for $5.
Check spark plug wires and insulators for hairline cracks. The coils are potted in ceramic and may need a look-see as well. This is most likely NOT your problem though. Spray some WD 40 on them then wipe it off. The cracks will weep and give themselves away.
Set the timing and re-do the points, make sure the prop is in good shape. (for balance. The vibration a bad one will cause is unbelievable.) You should be good to go.
Don't worry about overheating. As long as a solid stream of water comes out of what my wife calls the "PeePee hole" you are good to go.
One caution. Do not EVER run that motor without a good water source to the impeller. {I see idiots doing this on boat ramps all the time) You can burn it up faster than you can say "$90 dollars an hour for labor"
It took less than 15 seconds (and the wife) to shred a brand new impeller on our boat. Pulling a lower unit off a 225hp Johnson in a parking lot is NOT fun.