Zero turn caster wheel replacement problem.

I need a little help in replacing these on a Bush Hog zero Turn. I've just about had it with problems with these. The original tires are 11 X 4.00 - 5 on a 1/2" bore. And they don't have a grease fitting. The originals are a few year old with only about 150 hours on them. These have been easy to go flat and now the bearing are failing. Really just cheap designs. Bearings for these are in the mid $25 range EACH with two on each wheel.
I can get flat free tires on wheels from...

Zero turn caster wheel replacement problem.