The 350 and 450 movers are a darned good cutting mower, but the wobble box is a weak point, and often fails badly to the tune of lots of $$$$$$.
There is a pin (Key #5) that that knife drive lever pivots around that is clamped into the base of the wobble box casting.
90% of the time the mower has got to this point, the clamp area in the casting is wallowed out or broken off. A new casting, key #13, part #AE31780 is over $700.00.
SURE hope your's hasn't gotten to that point, and that a new pin (#25), bearing (#29) and a couple of bearing cups (#5) will fix it up for you!
I have 5 mowers with that same drive (#450's) laying around, with the casting bad in all of them!