Mate of mine actually fitted a Astra 16 valve motor but was not happy with the looks. Then he considered a S2000 Honda but very tall.

Finally fitted a BMW K1200 head onto the A series block and that gives ample power.

Only just seen this thread.

850 Automatics are quite rare now so from a classic car perspective better to keep it as it is and maybe sell it and buy a 1992 on Mini auto which were all 1275's (carb at first and later injection). All the early autos from 1966 had uprated engines: 850 39bhp up from 34bhp, 998 44bhp up from 38bhp and 1098 56bhp up from 48bhp. The 1300 autos were 60bhp like the manual cars. The 850 auto box isn't rated to take the torque from 1098 or 1275 engine and would probably suffer from slipping 4th gear/reverse gear clutch. Also there were idler gear bearing problems with early gearboxes being too small and having poor lubrication. Easiest conversion would be to used engine/gearbox from an auto Metro (63bhp) and fit 8.4inch disk brakes !