Hi Michael,
So far as "modern" mowers are concerned (those made for standard American engines such as Briggs), the base mounting bolt positions and output shaft positions are normally standardised, so if your original engine was standard, you can fit other standard engines as a simple bolt-on conversion except for one point: output shaft length and diameter. If your mower originally had a British engine however, it may not be made to the standard dimensions. If the output shaft is not in the same place relative to the mounting bolts, you have a fairly substantial job to fit a standard engine such as a GX120.
If you look at the GX120 installation dimension drawings here, you can compare them with your original engine, whatever it is:
http://www.justgenerators.co.uk/pages/Honda_Engines/PDF/979.pdf
