It could be an intake restriction.
Yours probably has a filter - later ones did.
It"s visible if fitted and accessible from the cover on the left side - where the PTO shifter comes out.
I put the tractor on a side hill, or run the left wheel up onto an 8 by 8 block (and block the wheels well to keep it from rolling) to move the oil to the right side of the transmission. Otherwise a gallon or two will run out the lower bolt hole.
I place the PTO in the off position before removing the cover.
If you have a filter, it will be a metal mesh type about 4" high, 2" round, with a metal can type cover, and sitting in another shallow can.
The top cover is locked with safety wire to prevent unscrewing.
If the filter mesh appears clean, that"s not likely to be your problem.
If it is crudded up, note how the wire is placed to prevent turning, cut it and remove it. Unscrew the top and remove the filter. As I recall the bottom can will come out with the filter - try to bring them both out together, because if the filter appears dirty, there will be wads of crap in the bottom of that can, and it can fall down into the pump intake. Not a good thing.
Reassembly - screw the assembly back into place, rewire the top to prevent unscrewing. Clean the cover surfaces (the old gasket was probably destroyed when you took the cover off), and I use silicon bathtub sealant to act as a new gasket.
Place the PTO shift in the off position, and replace the cover with a couple of bolts. Try shifting the PTO to be sure you have that lever properly engaged in the shifting sleeve.
If that works, snug her up, and try the lift.