[TooManyTractors]

My suggestion to you would be to clean everything first, this way you can start from a clean slate and will know where any leaks are after running it for awhile. When i clean my 400s and MTD 990 i remove the seat pans to access the transaxle. I park them somewhere i'm not worried about slinging grime and oil, then pressure wash everything. Next i spray engine degreaser where it looks like it's needed. Then pressure wash again. The degreaser doesnt need to be done every wash but if that old girl hasn't been cleaned in a few decades then a few bucks of degreaser will pay dividends.
Next, start watching Mustie1 videos on Youtube, he has plenty of videos focused on mowers, tractors, and small engines in general. His videos and funny narration helped me get started in understanding how our machines work.
Best of luck!

 

[TooManyTractors]

Good work, thanks for bringing us along!

 

[TooManyTractors]

I use a razor blade to remove gaskets, i have a gasket scraper as well but for small gaskets the razor blade works very well. You have to be careful as the razor blade is very hard and with too much force can mar the mating surface.
I cannot comment on the seal, to split the case or not, but usually small engines are not like car engines where a bearing cap has to be removed to replace the seal. Usually a seal puller can pry the old seal out over the crank snout. Make sure to debur and clean the snout before the new seal is oiled and slid on.
Keep it up, looking good!