There are several aftermarket BB engines available for the Grom. Most of them are direct bolt on motors that use the stock wiring etc. hondagrom.net is the best place for all things Grom. I am on their forum as I am considering a Grom come spring. The biggest direct bolt on engine I've heard of for the Grom is a Finbro 183cc. They are pretty hot little motors apparently. There have been a few crazies who have swapped in a CBR 250 motor, but it is a lot of work. Personally I say if you're buying a Grom, buy it for what it is and not what you can Frankenstein it to be. Once broken in the Grom will do 105km/h in stock form. (A few km/h less depending on the size of the rider) The bike will happily cruise at 80-85km/h all day according to those on the site. To me that is plenty for an in town bike or even for rides out of town in the country as long as you're not in a hurry. Honda designed these things knowing they will spend most of their life at WOT so for me, I wouldn't want to monkey with the reliability of the Honda motor just for a little extra speed.