Yes, it can usually be done on the tractor w/ the pump still attached..
There are MANY posts & video's on it.. some successful & some not..
Just google, CAV DPA top cover leak.. or.. CAV stuck metering valve..
THAT'LL keep ya busy for awhile..
They sell a mini gasket kit on ebay that has all the gaskets for JUST the top cover..9-12.00 w/ free shipping.
I suggest getting the gaskets BEFORE u lift the cover & try to fix it..
Some of the time orings wont fix the leak.. the shafts are steel that move in an aluminum top cover..
Dirt or other contam. get in the shafts & eat a groove on the cover..
So when u get the cover off & the shafts out, look to see if there are any grooves in the bores..
If yes, u can try the new orings.. u have nothing to lose.. but if it doesnt stop the leaks, I can put teflon bushings in them.
OR u can buy a new top cover..