A simple material stress check of all the components of the front end on the N series shows the maximum stress occurs in the center axle part at about 2 1/2" from the vertical centerline.
Since the part is sand cast steel, the allowable material strength is reduced compared to cold or hot rolled steel or forged steel.
Doing the numbers based on the knee extension in the most narrow position, the front end is good for about only an extra 420# of static load.
With a front end loader the extra weight of the steel frame, bucket and payload is bouncing, which creates a dynamic applied load rather than just a static applied load.
Replacing the center axle with a 2" X 4" X 1/4" wall structural steel tube would increase the allowable dynamic working load to ~ 1475#.
The gap in the center support weldment is 2 5/8".
Believe the a 2 1/2" X 4" x 1/4" structual steel tube will fit.
Using this size of member, the allowable dynamic working load would increase to ~ #1840.
The center king pin has slightly more than 1" sq. in. of cross section. This part is far from the weak link, stress wise , but wear on this part causes undo stress on the radius rod, as witnessed from bent radius rods.
A typical front end loader scoup holds ~ 1/2 CU YD volume.
Some research indicates the weight of dry gravel is ~ 2835 #/ CU YD and maybe more depending on the water content.
So, 1/2 CU YD weighs ~ 1417 #
Based on the design of the front end loader, only some fraction of the loader weight and payload weight is taken by the front end axle.
If 100% of the extra weight is taken up by the front axle,
This would make the original N series tractor totally worthless to load gravel, maybe compost.
If reto fitted with a 2" x 4" x 1/4" steel tube, would be marginal to load gravel
If reto fitted with a 2-1/2" x 4" x 1/4" steel tube, might be acceptable to load gravel.