If the ball bearings are in good shape, clean them up, repack, and keep on truckin'. They ran untold millions of miles when these trucks were new, and they were loaded down like real trucks then. Yes, the felt seals will wear out spindles, but I believe the inner ball bearings have an extended area where the seal runs, making the seal surface replaceable, plus that bearing race is hard as woodpecker lips! I don't know if there are drop-in replacement roller bearings available, but a little time spent with a master bearing catalog and a dial caliper might allow you to come up with suitable replacements if you really want to do an upgrade. Ditto for lip-type grease seals. It's really not necessary, however.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!