Not sure - IF it's the same gear box arrangement as the Maestro (NOT the 'in sump' of the mini) then it's possible, but it'd take a lot of work/parts/machining/expense. Can't believe it is though, as I'm sure we'd have heard all about it before now if it was.
Mini/ Metro engine problem is the rear main bearing seal - and crank of course. Maestro engine has slightly different seal problem - it could be useable - one of the suppliers is working on/has already a kit to use this engine - but not inexpensive! Since many want to use a Ford 5 speed gearbox - the obvious answer is to use the complete Ford engine + box - because it's the special A series/Ford gearbox bellhousing that is very expensive. So - avoid that expense and use complete Ford unit!!
it really depends how much modifying you want to do - the mg midget engine and gearbox is the easiest as it is almost a straight swap for the old engine. But the capacity increase is not huge, so you will not notice a large increase in power. look at http://www.minormania.com if you want some more radical ideas!
1969 Four door Saloon Old English White 1275 with ported head and HS4 carb. Wolseley 1500 front brakes. Currently off the road with a leaky master cylinder!