This place sells Tecumseh speed parts: http://hillsboro.net/users/dare/dare04/index.html
I didnt find any offset keys listed on their site but I'd imagine they sell-em for a couple bucks and could give you some good advice on how many degrees you should advance your timing to match the rest of your engine mods. (if any) I heard its faster to call this place than email...
Yea I'd call David. He doesnt always respond to emails. And like everyone said, it just improves throttle response and gives you a little more throughout the rev range. You just need to be careful not to shave too much off. What your actually doing when you increase timing is making the spark plug ignite sooner. Too much timing results in pinging and then you'd need to run a higher octane fuel. But Im sure David will tell you what's safe.