Hi,
Saw that car on eBay. There's a thread about the car somewhere on Ausmini.
No idea how mcuh work it would be convert back to a Mini subframe, we need to see what was modded in the first place. Post up some close up pics.
An 1100/1300 A series engine might bolt into the 1500 subframe with the mounts from a Morris 1100/1100S/1300. But personally I'd go with the 1500 engine or something more radical. You should be able to presuade a 4AGE or $EFTE Toyota engine into a 1500 subframe, as they have heaps more room than a Mini one.
Either way you are going to have rego hassles and require an engineers certificate (mod plate), unless the car was already mod-plated for the subframe swap.
Cheers,