Hi Ut1205, welcome to the revolution!
My first bike was purchased for $20.00 @ a second hand store, a Sears 1967 Fleetwood all chrome with three speed stick shifter between the legs. Next to mint condition. Three months later I purchased a Hawthorne with a three speed thumb selector shifter for $20.00 @ a different second hand store.
I decided to motorize the Sears so I swapped out the stick shift for the thumb selector shift so I could mount the gas tank. I have three speeds @ anytime. I like that because I can take off easy then work my way up to speed then pop the clutch to go. I weigh 185-188 lbs. and I have to assist the bike or it will bog down too much up some steep hills. The top speed on this bike with the 49cc (44 tooth sprocket) so far is 31.7 mph on flat asphalt. I'm not looking for speed, just reliability. I get both with this motor.
Last March I was in Florida looking @ bikes outside a pawn shop when a transient approached me and asked if I would buy his bike for $20.00. It was a Huffy fifteen speed Mountain bike. The rear wheel was bent and the seat was tore but other than that it checked out so I bought it. Four months later and some elbow grease and its been reborn with 68.5cc engine on it. I put the handle bars up as high as they would go and the seat down as low as I could get it. This is comfortable for me as I don't like to ride with head down and Butt up in the air.
The performance on this 68.5cc engine is remarkable. Up the hills with no problems. Throttle response is instant and top end hang on. I did have to remove the derailer located below the seat because of a motor mounting issue but I still have five speeds @ will when I'm riding. But that doesn't mean I can't use the other ten speeds.
All I have to do is stop the bike and slip the chain to the next drive sprocket and I'm good to go. I thought about a jack shaft but I wanted simple and thats what I have. It all comes together and works nicely. Here's some pics of the Sears and Huffy setup. Have fun Ut1205!