Well, I left the transaxle to drain overnight. This morning, I replaced the gerotor housing, removed the vent tube nipple, and put in Lucas full synthetic 20W-50 until it started coming out the level-check hole. I used about 2.75 quarts, which means there was probably around 1 quart of original fluid left (capacity is just over 3.75 quarts).
The vent tube was somewhat loose, so I added a hose clamp.
Then I jacked up the rear of the frame and supported it on jack stands. It looks somewhat unstable, but there's not much weight left at the back without the transaxle.
I positioned the transaxle under the frame and began lowering slowly.
Once I got everything close, I looped the drive belt over the pulley, reconnected the brake/clutch spring, and installed the freewheel lever. Then I set the frame down on the axle and bolted it back together. The last step of reassembly was the ground speed control linkage.
With everything back together, I started up the tractor and completed the purge procedure from the manual. When I got to the part of the procedure where you purge with the freewheel engaged (normal operating position), I noticed a problem. The tractor didn't move under its own power.
I removed the level-check plug, and very aerated fluid came out. It looked opaque, like there was lots of air in the system. Obviously something wasn't right.
I let the tractor sit for a couple of hours. I returned, purged again, no result. I removed the cap at the end of the vent tube in case it had become plugged and was preventing a purge from working. Again, purged with the same result.
At this point, I'm thinking that there's lots of air in the system, and for some reason the purge procedure isn't working to get it out. I'm starting to think that draining the fluid by dropping the gerotor housing isn't the best way to do it. I might end up pulling it out, refilling (maybe upside-down through the gerotor hole), and bench purging it. We'll see what happens when I try it again tomorrow night.