Someone was looking for an M5 engine awile back. I saw an engine for a e34 M5 for sale on ebay at the following link. This engine would probably fit in a e28 M5 as it is very close to the same size. Don't know this for a fact, but would guess that it could be made to work and would be stronger as this engine has more cubes and horses. http://cgi.ebay.com/ebaymotors/aw-cgi/ebayISAPI.dll?ViewItem&item=590992975&r=0&t=0