Biking is a good option for you knee to help get ROM back.
Is your ROM getting better? if it's getting better and continuing to do so then go with it for now, only be worried about it if your getting nowhere. - most surgeons want you at 90 degree before they are happy.
Isnt ski season pretty much over? where are you skiing? - sorry I'm in the north and don't ski to be fair. Probably a good idea to visit a physio that is a skier? are u doing that.
The fibula isn't weight bearing really so you would be and about on that very quickly - the femur is a much stronger bone and takes alot to break and hense takes alot more time to heal... I would say that at 13 weeks you may be ambitious but I don't ski so don't truely understand the amount of force required.