Enginge stroke.... 2 or 4

I like the 4s for sound, umph and the speed. Down side is they are not fun to pedal if ya have to. Just too wide, inframe.

Both types have their pros and cons...
The 4 stroke is going to be bulky and a bit heavier... the 2 stroke is much more compact but ya gotta mix the fuel.

A 4 stroke has better low speed manners, lots of low rpm grunt... a 2 stroke has better high speed manners, not too strong down low but once ya get the rpm's up they can be very beastly depending on how it's tuned.

The 4 stroke has a much wider powerband so you can have good power down low and still have excellent top end power, the 2 stroke can be tuned for low end torque or top end horsepower, but it's difficult to get both from a 2 stroke.

a 79cc Predator engine will get you around 30 to 35 mph, but for the price, the 212cc will Easily get you over 40 mph stock... and is capable of double that in full race trim, so for the price most people opt for the larger 212 and either leave it stock or do the milder mods like removing the governor and adding in some heavier valve springs, which should be enough to get around 50 mph when used with a CVT or geared high.

I'm not very big into the 4 stroke engines, but others in here can better guide you on what size and what mods if any are needed to hit the cruise speeds you're after. The 4 stroker will also be more reliable, just generally speaking as there are a lot of 2 stroke China Girls that can go over 5k miles trouble free, and the Morini, KTM, deNardi, and their clones will last as long as any decent 4 stroker as long as maintained properly.

Another plus for the 4 strokers is that they can run a lot quieter since they can cruise at a lower engine rpm, 2 strokers can be silenced but they still like to be reved high for power... Generally speaking.