One of the halogen lights on my hood fan just stopped working. The existing fan, and the enclosure, mention that it should be a
GU10 120V 35W light. I replaced it with a
GU10 120V 50W light.
Is this safe?
Probably, but you don't want to.
As comments mention, there are several factors which play into the maximum wattage. You don't know which factor(s) was/were the driving factor(s) in that rating so you have no guarantee.
The consequences of ignoring it could be as benign as a fuse breaking, or as bad as the insulation cooking and releasing toxic chemicals which cause health effects, or just getting it done with and burning down your house.
Buying the right bulb is probably a lot cheaper.