Engine Mounting Bolts

[manicmondays0]

My Japanese 25HP Mariner has two bolts which seem to be the main engine 'mounts' they run through a thick rubber washer and have a nut on the outside to hold them in place. One of the bolts has a shredded thread and so the nut is holding on rather more tentatively than I would hope. Can anyone tell me how to replace these bolts from the inside and whether it's a relatively straightforward job or is it one for my local engineer?

THanks

Simon