First off, thanks to all for the fast responses. It really helped putting these symptoms together into a diagnosis. As soon as I got everything back together, the hydro gears started doing their job again. WOOOHOOO! It feels good to know I didn't make a $1600 mistake. Attached are pictures of the two offending fasteners (nuts and bolts).
There is obviously some corrosion going on there on both bolts that failed, but I can't say for sure it was a significant contributing factor in the failure. The two unbroken mounting bolts, however, were in much better condition. I replaced all four to make sure. I used longer bolts, spacers and lock washers to keep the thicker shanks of the bolt in the areas that are probably most prone to damage/breakage. I ended up using grade 8 bolts again due to local availability instead of stainless...Also, I read that standard stainless bolts are not good for use in a suspensions because of their lower tolerance for impact.
The cross brace wasn't cracked or bent, so I decided to re-use it for a little while longer...at least until I can get a replacement or until I can engineer a reasonable fix for the current one. If I modify the current cross brace, I will post pictures.
I also have found what appears to be a "reasonably" low-price source for the updated cross brace, which I may end up buying anyway. The shipping charges keep it around $100 to my door, but I am also trying to negotiate a freebie from the local dealer that originally sold the mower, but that's a longer shot.
Has anyone bought from this place (below) before?
https://www.hustlerlawnmowerparts.com/hustler-part-123292/
By the way, I was able to find a 50.5" X 0.5" drive belt at our local APC (Carquest) for $18.18. The belt was an ag/industrial belt that was fabric-wrapped. I cut grass for about 30 minutes after the repair and everything works great...so far. I also ordered an original 607400 belt the day after my first post for $24.95 in case this belt fails.
I will update if anything changes in the short term, as I plan to do some serious mowing this weekend.