The seat is a NOS Italian made 70's chopper seat. Found it on eBay and just modified it to fit the bike. I don't know about without your exhaust but on a 2 Stroke motor you really need and exhaust to run right. The one on this is a Bullet moped expansion chamber and really seem to give some extra punch. I don't know how fast it will go yet as I am still breaking it in. I did give it a little goose up to 32 the other day. It is so hard to keep from giving a good long twist.
I am not sure what size it is. I got a pipe and the engine mount together then cut down the pipe and stuck a bullet made for a PUCH moped on the end. They use the same pipe for the 49cc or 70cc on mopeds.