When you say By design it would have been LHD I take it you mean the "kit" from the US?
If so the front subframe is the same LHD or RHD only the rack is different. The only issues you have with putting the LS1 in the RHD RX7 are custom exhaust manifolds (to get around the steering column), a modified clutch pedal/master cylinder as the oem version wants to share the space with one of the cylinder heads and relocate one of the coil packs (just a tad) as its far too friendly with the brake booster. Apart from that, easy. In fact things like power steering lines etc are actually located in a much better place than they would be on a LHD.
Just to give your MX5 customers something to think about, have a look here