boaty boatman

Access is required to the engine - if anything drops inside ( which it probably won't) retrieve with telescopic magnet stick (couple of quid from shops and ebay) If you have any kids they would like retrieving the nut - bit like hook-a-duck!

To do this properly you need to remove the gearbox and fit the new starter motor to the engine, then refit the gearbox. To do it cheaply ( ie bodge), drill right through , and fit nut and bolt or get nearest size thread ( M8 or M10) from B&Q that will go all way through. Fit nuts at each end. Use felt washer and steel washer on casing side. Personally, I would remove gearbox and fit properly to engine as it is quite easy to do ( even for the novice) and get a recon starter ( rather than 2nd hand or scrapper) And if you remove the box, you don't need to cut bolt head off, as you will gain access to the nut

Once old bolt out, drill hole right through casing and fit a new, longer nut and bolt

Cut the head off the bolt with a small grinder, remove starter motor then remove rest of bolt