If you weld a plate over it it will possibly hang you up on those sapplings as they wont be allowed to bend to go under it. It may rip the trunk of them may pull them out of the ground or may just hang your tractor up as you may not have enough traction to move once hung. I have a ym2000 and unless you ballest them there not very heavy tractors and thus they lack good traction, the mower will hand you up sometimes if it high centers!!
I would not weld a plate personally. I have cut some heavy stuff befroe with my Jbar economy cutter, nothing really hits you?? It throws some stuff foward right of the tractor but you havve to thing its under the deck as your moving foward, not only does it have to go out foward but up to get to you, now it can counce off the arms and stuff to hit you but i dont think i was ever hit with anything other than small swig size stuff.
What i would do if you want something is i would weld an extention of the top of the deck onto it, say a piece of angle iron like 2x2 and then drill the face of that and put on something flexible like the conveyor belt or old truck mud flaps or what ever you have that will get you even farther from the rotating blades. If you need more space i would get something longer but i would not want something that hangs to low like a 2x3 angle would be better putting the 3" side on the top to extend you farther out but not going to much farther down than say 2"s as it will hinder the ability of brush to be pulled under.