[Conkers]

Sounds like you'll be ok, just smoothing things out and cleaning it up will be a good start.
I would spend the money on changing the guides and getting the seats cut with three angled tooling, these two things alone will give more improvement than any light DIY porting.
Don't go silly with the porting, just give yourself 5-10 minutes on each port with polishing cloth or 30 grit stone, and for the DIY-er NO rotary files. That way you'll clean it up without doing any irreversible damage!
Best to remove the valve guides first, clean it up and then have new guides fitted, seats cut, valves ground, vaccuum tested. Vacuum testing is better than lapping, lapping is old school (not necessarily a bad thing) but even with a the fine lapping paste creating a fine grey line and marking blue, you can leave particles of lapping paste in he seat, which will hammer the seat and widen it during initial running.. Then skim, volume and equalise the heads and you'll never have to remove them again.

Good luck and dont hesitate to ask me if you need any more advice.

Mike.