Unless the casing is cracked it should be possible to rebuild it - its all standard Bosch components inside.
They are, or at least, they were. I rebuilt a 328 starter for a guy a couple of years ago with such a parts kit. I do not recall any particulars - it wasn't my car and they guy supplied the Bosch parts kit. I'm sure the starter was used on other cars as well. Though I can't say it for sure, it seems EXTREMELY unlikely that a one-off starter was designed just for these cars.
Well, Im usually a lazy one, especially on Friday afternoons, but this is the break down by parts references that I have noted back in 2011, with the 2011 prices: Solenoïd is 2 339 303 286 (64) Main kit is 1 007 010 039 (26) Bushings 1 004 336 404 (45) Bearings 1 000 301 056 (3) Crown gear 1 006 209 528 (44) Fork 1 001 933 111 (1.50) Rgds
Is this the beast? New IMI Starter Motor 1990 1991 1992 Ferrari F40 V8 2 9L 0 001 110 004 120967 | eBay
Nope: that's an IMI, not an original Bosch. It Is presented as a direct substitute, but I can't vouch for its adaptability. It doesn't look like the original one, but That does not mean it can't be substitued to it, of course. Rgds