Charles, you can get the 125cc semi-automatic engine, however I'd suggest that you stick with the 50cc.
Why go with the 50cc as opposed to the 125cc, well for one you'll probably destroy several rear wheels unless you use a rear wheel from a mini 50cc/110cc kids dirtbike. Regular bicycle wheels just can't handle the torque coming from these Honda Monkey Bike chinese cloned horizontal engines.
You might want to do a search for the Kikker Hardknock Bobber wheels, the rear wheel is 15" and the front wheel is 18".
I myself am contemplating using these same wheels, as the are surprisingly close in size to the Schwinn Stingray OCC Chopper rear wheel, here's a pic of my bobber. If you use these wheels you'll also have to have the axle slots widened as this axle is approximately 1". And the tires are speed rated as well.
If you get the semi-automatic you can add a suicide shift, remember semi-automatic means no manual clutch, this is my 110cc bobber semi-automatic pictured below.
Just take your time and do as much research as possible before you start your build.
See two other forum members builds using this type of engine, they are Civlized, and i-paint, both have way more experience with these builds than do I.
Here's the link to Civlized, when his profile page opens click on the Statistics it will show all his Post's & Thread's here in the forum. Then just click on the ones you want to read that are related to this style build.
http://motorbicycling.com/members/civlized/
Here's the Link to i-paint, when his profile page opens click on the Statistics it will show all his Post's & Thread's here in the forum. Then just click on the ones you want to read that are related to this style build.
http://motorbicycling.com/members/i-paint/
As for the color of the engine, you can use high heat paint that is specifically made to paint engines.
Hope this helps!
Peace Crazy Horse.