My old Westerbeke 4107 (aka Perkins 4-107) has a Lucas starter, also, and the last time it had a problem, the guy at the starter shop told me parts were getting scarce. Fortunately, that time it was in need of a couple of bearings, new brushes and turn the commutator down, so I got off (reasonably
I did a little searching around, talked to some friends, and found out that I could buy a whole engine for what it would cost to replace the old starter motor from The Prince of Darkness (aka Lucas--ask anyone who's ever owned an English motorcycle). Rather than hunting for just a starter, you might want to peruse the used market for a complete, running engine. I've found them for $300-$500 bucks, complete with transmission and v-drive. I think I'd go that way, strip the pieces that are expensive (starter, injection pump, injectors) plus any of the other weirdness that Westerbeke chose to put on your engine, have that starter rebuilt, and save the other goodies for a stormy day.
BTW, some of those old transmissions, if they are in good shape, are worth more as parts than the whole engine/transmission ass'y is. Go figure.
Last thing, check with a good electric motor shop. Old timers in that business who don't mind getting clever can and will find suitable field windings, rewind the armature, and all of those other things that young kids don't know how to do, or are simply too lazy to do.