Hi Sparker, i bought one new a couple of years ago and it cost me about $32 from a local Rover dealer.
I've also fitted GXV160 starters to them but it's a little more mucking around. Use the same blower housing, cut off the old starter and bolt the GXV starter on making sure you use the right one as there are 2 types available, the one with the flat pawls and the one with the round pawls, you need the one with the round pawls. Use pan head 1/4 inch bolts and put them in from the bottom up, and finish off using nylocks. Putting them in top down will see the nuts hit the fan on the flywheel.
Now that i think about it the spool, spring, friction plate, pawls, etc may fit from the GXV starter directly into the rover starter; try it, and if it works let us know, otherwise put the guts back into the GXV starter and retrofit it as outlined above. With tyhe above process you cannot refit the original dress cowl over the new starter. The GXV starters are cheap enough on fleabay, $10-$15 and work well as a cheaper alternative to the genuine.