I completely agree with Dave here, a 36T should do you a great job, Im over 200lbs and I run 34T, 36T, 41T and a 24" tire bike has a 30T all of these bikes are capable of 40+mph and they pull me up hills just fine and get up to top speed in what I consider to be a respectable amount of time.
At 105lbs if the engine is strong along with the gbetter tune you have now you could probably get away with a 32T, but the engine has to have the grunt for that size sprocket and just having a good tune isnt enough for it, higher compression, good exhaust, good port work and good overall balance of the engine is a must whenyou get below a 36T in my experience.
You being such a lightweight is to your advantage big time though so like Dave mentioned you can get away with more than us 200+pounders can.
If your bike has a typical size coaster hub you can get the whole setup from MZmiami of ebay shipped to your door for $50 or gasbike.net has them for $45+shipping, MZmiami also has the 32T sprocket set up if your engine is up to that task if you want a really fast top speed on the flat, but I personally recommend a 36T and then you could always spend $25 later on for the 32T sprocket only if you feel it pulls the 36T very good and has some power to spare.
Best wishes with whatever you decide.
Map