I've done some of it in my Land Rover! There's boulders and gates to prevent 4x4's (or there was a couple of years ago) part way along. Some of the river crossings got washed-out and the farmer (so I am told by a mate in Aviemore who's also a Defender enthusiast) took the opportunity to block access. I am sure you could get a bike through the road blocks. My news is about 2-3 years old tho.
Even the parts before the road blockages are spectacular, and deserve to be done, the scenery is amazing, and both times I have been, I got very close to larger herds of deer
There used to be cycle/running races over it, so bikes (of some widths) can certainly get over