If I remember correctly, the older tractors had rear hitch arms that mounted in only one position, and the optional engine guard extended over and behind the hitch bracket.
But when the the G series came out with the black engine guard that bolts inside the hitch frame, a new two position hitch arm was introduced. This allowed the hitch frame to mount in the original forward position for mounting the rear tiller lift-able hitch, and for mounting the bagger/vacuum blower assembly.
But these tractors came with the hitch arms mounted in the rear position to allow the new style engine guard to clear the engine.
So look at your hitch arms and see if they have two holes where they mount to the axles , or three holes, with the middle hole being elongated.
If they have three holes, move the hitch arms to the holes that move the hitch back, and a new style engine guard should work.
Because I have the vacuum bagger, and did not want to remove the hitch arms or the drive pulley when the bagger/vacuum is off the tractor, I did this:
I had my metal guy make special extension brackets so the hitch frame could stay in the forward position for mounting the vacuum, and so that the engine guard would be far enough back to clear the pulley.
You need hitch arms like this:
If you are trying to use an engine guard that looks like the one on my tractor, or like this:
The new style hitch arm is #047529, new number 08780853
In this picture you can see the extra forward hole of the hitch frame in front of the axle.