What bits are you having problems with?
I have not assembled one yet, but looking at the picture on page one here, and then looking at the components and holding them together, it seems OK. Just have to line the bits up when glueing (and avoid fingers) and clip/bend the pipe for the watercooled MG before sticking.
Might be best to stick the motorbike to a base first, then stick wheel to sidecar, then sidecar to motorbike