i say just scavenge for dirt around the property. to build a ramp. but you will need to compact as you go. so things do not settle. if a flash flood nails you, ya it may take out ramp, but the rocks should still be there. other words just build a dirt ramp, as if you were going to build a dirt drive way up across and into the back yard.
a temp setup... you are just asking for $$$. if you used hard wood boards. you would be slipping all over the place.
perhaps buying 2 or 3 sets of some long trailer ramps. and then welding an extra piece of metal both sides. to connect ramps together to make them longer. the extra piece of metal the longer the better. to help reduce bending.
get a old trailer off of craigslist. chop off the wheels, axle, and springs underneath. and perhaps chop off the A frame on front of trailer. and some how winch it up over the rocks. you might need to add boards say 1"x2" boards across the trailer. to act like a grip for tractor tires.
a TLB (tractor loader backhoe) could make it up. if newbie to TLB. it would most likely be a scary ride, working backhoe and FEL and going forward. and pick tractor up off its wheels and walk it up over the riprap.
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what are you wanting the tractor in back yard for? just to park it? or you wanting to do something?
there are many options, to small size mini's that can drive through 30" doors (door comes off along with hinges). (assuming you do not have basement) could possibly drive through house, to dig. or like.