Depends how basic the frame is I guess. As you say. it'd have to be made for a particular front end, or at least a standard headstock bearing size. If they just supplied the basic engine cradle with one length hardtail, clamping on a regular Reliant axle & left you to add tank, seat & footpeg mounts etc, it might work.
Hmm, that still leaves a lot of welding work to be done. Presumably these frames get bought because people dont have the ability to weld in the first place? Certainly for the sort of money the sellers ask, i`d want a finished frame, with everything there, ready to bolt to. A basic frame can be made for around £200 add on some profit and your looking at £300 for a basic frame which would only include engine and gearbox mounting points.
Add the same again for engine, box, axle and a front end, £50 to light it up, £??? for a tank n seat, then all the odds n sods aswell.
Your lookin at £800 - £1000.
Cor, this trike building lark gets expensive when you think about it!