Here is the overdrive link Mark
Roverdrive Gear Corporation - Home
I was thinking about your case whilst driving to Perth yesterday and wondering if you really wanted to change the engine, and it got me thinking about a chap in a magazine who put an Eaton supercharger on his 2 1/4 litre petrol. This gave the oomph and I am pretty sure he used an overdrive as well. The Rover engine would lend itself well to a blower and running on LPG. Eaton M45? blowers come up on ebay quite frequently, as Mini drivers have theirs' upgraded to a bigger blower; the ones I saw were quite cheap but the overdrive is not :(
Cheers Charlie