The trail from Kilnoch Hourn is a great walk, but not really conducive to a pleasant bike ride. The route is boggy and rutty – you’d probably wind up pushing most of the way until you get to the bothy at Barrisdale.
Sometime ago (2-10 years?) on of the UK MTB mags did an article on a route in from, if I recall correctly, the end of Loch Arkaig. It was an overnight ride, with a stop in a bothy on the way, then ferry from Inverie back to Mallaig. From what I remember, they wound up pushing a lot as well.
Done a bit of riding in that area myself, don’t really recommend a ride in to Knoydart. Do some walking there instead!
For riding in that area, we have linked some multi-day rides together in the Glen Affric/Glen Cannich/Attadale area just North of there, with lots of good riding. Be mindful of stalking season though.