You shouldn't have any trouble with the engine you ordered and should easily hit the speeds you're looking for without any mods. Just take your time on the installation and don't get in a hurry with anything, get it running and broken in then tune the carb by jetting it down until the 4 stroking stops. Even the stock pipe can perform if you open up the outlet either by drilling more holes in the end cap or by cutting off the undersized outlet off and welding in a larger one. An expansion chamber pipe will get you up into the 40mph range for speed if you get the right one, or really good acceleration if you go bigger with the rear sprocket to keep the max speed closer to the 30mph range.
For porting, this is a huge topic area, but just cleaning up the flash around the port openings and smoothing out the casting ridge in the transfers will make the top end last as well as make your engine run stronger and smoother. Clean up any overlapping nikasil plating before you run it, this is where most engines fail early on because this overlapping plating can lead to it flaking or peeling off.
If you want more power after it's broken in, just make your intake and exhaust ports 1mm wider on each side, then bore out that shorty intake to match the carb. You can pamp the pistons for more transfer and exhaust duration which will give you more rpm and top end power rather easily, but anything further than this, be sure to read up on since you can go too far and move the power up higher than the engine can rev to.
