So, I went with the non-suspension compact tractor seat "Black Talon" seat sold at Tractor Supply. The main problem was not the brackets, but the 3pt hitch lever which the seat hit. I bent the lever handle as much as I could, then bent the trim flange on the seat, where the lever hit--using a deadblow hammer. This combination got me clearance.
There was already one bolt hole on the old Kubota seat brackets which I cut off the old pan. This hole is perfect for attaching the bracket to the Talon seat, except that the bolt head prevents use of the middle of the three pivot pin adjustments because it impinges on the bushing holder.
There are two places on the seat bottom the bracket will bolt to. I experimented. I ended up using the most forward bolt hole pair, with the rearmost pivot pin location. That is about a middle position.
It is a very nice seat. If I were to re-design it specifically for this tractor, I would take a half the height off the seat back. I did not look to see if a low back version of the same seat were available.
In summary, I did not need any adaptor plates and did not need to drill any holes. I re-used the brackets which were on the stock Kubota seat pan, so the pivot and functionality is the same. The only modifications I made were for clearance for the lift lever over the right fender.
I used two bolts, 5/16" by 1/2" long. with plain washers and Loctite on the bolt threads.