Re: Need advice...Great site lots of info...
Welcome to the forum. It's probably better that we tell you what we have and how well it works for us, rather than advising you on what to buy.
I have a 48cc 2 stroke frame-mounted on a chromo mountain bike. It carries my 200lb body on a 50 mile round trip commute at well over 100 mpg. However, AZ law limits me to 20mph (unless I want to get a tag, MC license and ride in the traffic lanes).
If you want to go 45mph, I would get a sprocket the bolts to a rear disk brake hub or use a hi-hat sprocket adapter on a rear disk wheel. It is difficult to install a clamp-on rear sprocket without some wobble. That wobble gets scary above 30mph.
“You're only young once, but you can be immature as long as you like" -Unknown