The first thing you will need to do is to drain the transmission of hydraulic fluid. There are two drain plugs.
Then look for the PTO speed selector lever. It is on the left side of the housing that your seat is mounted on. That lever engages a gear to operate the PTO. You will see that there is a cover plate with 5 or 6 bolts that will allow you to remove the cover. There is also a gasket under the cover so be gentle when you remove the cover. The PTO gear selector handle will come off with the cover. The filter is visible -(It sits in the middle, with a metal top and bottom cover) Remove the top cover by turning it counter-clockwise. Then lift out both the bottom cover and filter together making sure not to spill any sludge that may be inside the bottom cover. There is also an O ring that sits on the opening where the filter is installed. Make sure that you don't accidently dislodge this O ring and shove it into the hydraulic pump. It is needed so that the bottom cover of the filter will seal properly when it is re-installed.
Clean the filter (If you don't think the filter is cleaning up well buy new one)and the covers with Varsol and then re-install them. Then put the PTO cover back on making sure that the inside portion of the gear selector is placed in the slot so that it will engage the PTO properly, Then re-fill with new transmission oil and hope for the best.
P.S. This is assuming that your 1974 is the same setup as my 1970