Sounds like the wheel is, as you say, missing for the thicknessing bed height,
I've heard of people using a 3/8" drive socket for the spline, and making one up.
And also sounds as if you are missing the guard, there's one on Fleabay at the mo, in yellow, you'll need to extend the yellow bit a couple of inches as it's for a 6" cutterblock, A rare find indeed!
Regards Rodders