I agree with what people are saying.
You may just wanna get a two stroke. The grubee's are a little better than the rest from what I've heard, but still need a lot of maintenance for sure.
If you're gonna get a 4 stroke though, Bikeberry just stocked the new Grubee 5G (wish I had it!!)
4-Stroke Pull Start 48cc Bicycle Engine Kit EPA Certified BikeBerry.com
Definitely figure in a MM hub adaptor (mmbikeparts.com) into your budget because the one's that come with the kit drive the spokes, messing up your wheel, and are impossible to align anyway. They'll machine you a custom adaptor and sprocket.