I need a vehicle to access the lower yard and carry or tow stuff. The path is only 42" wide and 26° at the steepest. Will a 90-125cc 'youth' quad 34-37" wide do that?

 

Most youth quads are not designed to pull any weight, not geared low enough to do serious pulling. Many are geared for 200 lbs max weight period. Any way you could make that passage wider foe a bigger machine?

 

You should be able to squeeze a Honda Recon through it, the specs say its 40.7" wide 2017 FourTrax Recon Specifications - Honda Powersports . It would be a tight fit but should make it with stock wheels and tires. I don't think there's another adult sized atv that is any narrower than a Recon. Would be a much better choice than a youth atv which are only designed to haul children and no cargo at all.