Hi John,
I've built a several IC engines, a single cylinder hit n miss, a single cylinder throttled engine, and a two cylinder throttled engine. The hit n miss engine was my first and I was pretty lucky with it. The only problem I had was that my battery voltage wasn't high enough. I was trying to use a six volt battery with a motorcycle coil and regular points. It wouldn't run so I finally bought a 9.6 volt RC battery and it worked great.
The next two engines presented a number of issues. I had a lot of trouble trying to get the valves to seat properly. They have to be a dead nutz seal or the engine won't run. Good compression is vital and in both cases, I used an o-ring on the piston. Once I got the valves to seal the o-rings gave me great compression.
The next problem I had on both engines was carburetion. I tried a vapor carburetor and had reasonably good success, but I couldn't get the mixture rich enough... maybe too many air leaks in the intake system. I've tried various carburetors, again with mixed results. It seems that these two throttled engines are easier to make run good at low speeds if you have a load on them. For that reason, I'm adding a generator and a 60 watt light bulb to my 2 cylinder engine.
I've never tried a hall effect ignition. I have used transister ignition but used standard points to make it fire. Ignition is something you'll have to deal with. There are some good alternatives for under $50 if you want to use electronic ignition. S&S engineering has a unit which includes the coil for about $50. I've used that one on my horizontal single cylinder engine. The unit I have fires on both the make and break of the points, but that doesn't seem to be an issue.
Chuck