First I hope I have this in the right place and second I hope it hasn't been covered to death. I have searched but I am not sure how to word my search so I might have missed it.
I have a homebuilt cnc router/carver that I have working quite nicely now after months of tweaking(still plan to expand and modify but future plans) it cuts nicely on flat wood and aluminum. What I would like to do however is be able to engrave and carve reliefs on non flat surfaces such as motorcycle engine covers, front forks etc.
Is there a way without breaking the bank to have an sensor of some sort to follow the surface and reset zro onthe fly? or something of a similar nature. I suppose I could sit down and model out each part I am working on and feature mill them but that would be massively time consuming with my rhino/solidworks/mastercam skills and still would miss anything I missed as far as curves go. Is there anyway to approach this for under say $500 or so?

An example would be say I wanted to cut a sheridan style leather pattern into the lower front legs of motorcycle(not perfectly cylindrical).