First of all, you have posted this in the Dirt Bike section by mistake. I have moved it to choppers.
Next.... the most difficult part of this is to build a frame with correct steering geometry. Often, modifying an existing frame will result in an unrideable disaster because of steering problems that are either unforseen and/or very difficult to fix.
I suggest getting a bare chopper frame made by a reputable shop. There are several such companies on-line. Once thats done, you can install the engine of your choice (decide on the engine before buying the frame so you know it will fit) and choose the brakes, wheels, accessories etc. There are a huge variety of such parts to choose from.
Finally, do some planning beforehand.... so you have a good idea of all the components you wish to use.