OK -- re-tapping it is easy. They sell individual taps at Lowes, etc. Just go slowly and keep backing it out to clean the cuttings (a little oil helps a lot - just regular household oil like 3in1 will be fine). No reason to worry because if you screw it up you can take it to somebody and have a helicoil put in as it takes a still bigger hole. Oh yeah -- definately go one size larger. If that is a problem with what you have to drill out on the part this bolt connects to the tapped part, then you will have to go with helicoils to keep the same size bolt.
This is not hard. You can easily do it.
For measuring threads take it to Home Depot or Lowes -- they should have a guaging set mounted there somewhere that you just screw the bolt into. Try each hole until it screws in easily -- that's it!