I'm sure you know, Mark, you must never use a spark plug of different "reach" (thread length) from the original one. If you fit a shorter reach plug (which had been done to this engine before you owned it) the spark will be occurring outside the combustion chamber and at best that will retard the ignition, but it may prevent it from firing at all. Furthermore the spark plug thread in the cylinder head will carbon up so you can't put the correct reach plug in without stripping the thread in the head, unless you re-tap it with a special tap - the thread is standard for spark plugs, but its pitch is intermediate between 14 mm fine and 14 mm coarse. It is even worse if you use a plug of longer reach than standard: the piston may hit it, with dire effects on the piston, and even if it doesn't, the part of the thread that extends into the combustion chamber will carbon up and it will become impossible to unscrew the spark plug without stripping the thread in the cylinder head. Short summary: use the correct plug, and when you obtain a second-hand engine, check the plug as part of your initial inspection before you start it.
The Honda carburetors are a bit prone to gum formation, especially in the emulsifier lateral holes, and they will run badly at off-idle and intermediate speeds if you don't rectify this. The main jet also builds up gum, but moderate build-up seldom has much effect except for making them over-sensitive to choke setting. Unless you get the main jet back to its original size of about 0.55 mm, you'll find you can't open the choke fully immediately after cold start. I once put up with that for a couple of months with a GXV140, thinking it must be one of the effects of the US emissions calibration, but then I measured the jet and found it was 0.45 mm. I gave it a short blast of carburetor cleaner and it suddenly was 0.55 mm, and I could open the choke immediately after cold start even in the middle of the Melbourne winter.
