I had to alter the models and tweak the fpe files a bit:
The wheel barrow needs to be collisionmode=1 with issimobile=yes. Reorientated such that the origin is in line with the axle and 'forward' is +z.
The wheel needs to be rotated so the axle is through the y axis, collisionmode=3 (cylinder) and issimobile=no.
The script is really simple.
Note that you wouldn't be able to move the wheelbarrow around by script using this method, it simply allows the wheel to be spun for a bit of 'physics' atmosphere to the entity.
You could also add damping to the rotation to mimic a rusty wheel.
To make a more functional wheelbarrow you would need to use the techniques in this link https://forum.game-guru.com/thread/218283#msg2582026.
Btw, you can use this same technique to hang doors in door frames so the player can simply push them open!
Been there, done that, got all the T-Shirts!