I've had a look at the Omega as a possible donor.

The engines are pretty big but should go in ok, ditto the gearbox with similar mods to the tunnel as in the MT75 chassis. The differential is big too, but again doable. The rear suspension suffers from the same problem as the Mercs and BMWs - none of the links are parralel with the axle or the vehicle centreline, so arent really any use. Not a massive problem in it's self but the rear hub bearing housings are part of the enormous trailing arms. This means that you would need to turn up some housings that will take the bearings and bolt to the fabricated upright like the Sierra ones. A machine shop job but not too bad.

At the front the main problem is thjat the uprights steer from the rear, that is that the steering ams face backwards, putting the steering rack right where the sump/crankcase is on a Roadster. A possible fix may be to swap the uprights side-for-side but I haven't done any detail work on wether the suspension geometry is workable or if there would be problems with the brake caliper rotation, with the hub effectively running in the opposite direction.

I've heard there's another book coming out that has a fabricated front upright that might be used in it's place

Cheers

Chris