Hi there,
I'm a mechanical engineer with over 15 years experience in designing, modeling, prototyping and effective manufacturing of different products, ranging from consumer goods to alternative energy systems.
I could design your dirt bike, and for this I have the advantage that I've designed and manufactured a new type of go-kart (reverse trike architecture) and I have the support of one of my friend which is Romanian Champion at enduro (this guy is crazy about motos and dirt).
In order to clarify some details about your project you may open a msg channel here and we'll talk more about the fetures and specs of your bike.
How I work and what I'll deliver: based on your input (drawings/sketches, etc) I will design a rough model. On this model, with your feedback I will modify it until you'll be pleased with the outcome. After the model is set, I'll start designing the frame based on specifications (motor type, tyres, transmission, etc) and the other components making them to fit into the deasign.
Once the components designed (with frame analysis for torsional stiffness) and assembled together, I'll deliver you the CAD files for each component and sub-assembly in different file format (ipt/iam, iges, stl, step, etc) so to send them for manufacturing, 3D pdf so you'll be able to open/view the model into the regular Acrobat Reader, and the execution drawings (including BoM - bill of materials)