If you really want to get your Spark going faster it seems like you should go with the Vtech tuning kit. Some people on the forum have done this and there is plenty of information about it in threads if you search. Not sure about the solas prop that @serpent710 is talking about.
My buddy got the solas impeller and we raced I have free flow exhaust and removed the flame arrestor (not really performance mods in my opinion) and we were dead tied, we did race a stock spark at the same time and were just a little bit faster, so far I am not sold on the impeller due to these results. However I will still be purchasing one to see how much I can gain from it as well as reducing cavitation and corrosion as the stock one is already heavily corroded after only 14 hours.
Solas prop won't get you there by itself. I installed one and seen no change in top speed, maxed out @ 51. I have two 90 ho's w/ibr and it was marginally quicker than stock spark side by side. That stated it seems to have less cavitation out of the hole and a better acceleration from 30-45. If your looking for a speed machine a spark is not the answer, it's meant to be cheap and playful.