A few comments:
(1) In general, complete unit assemblies (such as mid-mowers and front end loaders) are not available for older Kubota tractors. I have never had any problems purchasing individual parts for the tractor or its attachments.
(2) I may be wrong, but I thought that it was around 1984 or 1985 when the Kubota came out with the
B7100HST. The
B7100 gear drive production started in the mid-late 1970s.
(3) At some point in time the wheel base on the
B7100 got extended. This is supposedly why the front end loader for the 1990s model will not work on the older gear drives. This extension affects the mid-mower availability. I was told that the extended
B7100 chassis came in the mid-1980s with the introduction of the HST models.
(4) The older gear drive B7100s used a front PTO with electic clutch and only had a 48" mid mower available; I would not recommend anything larger than 48" with the front-PTO on a
B7100. The 1990s B7100HSD had both 54" and 60" mid-mowers to mate up the the mid-PTO. Based upon your description it appears that the early B7100HSTs had a mid-PTO as well, but I don't know if it used the same mid-mowers as the 1990s
B7100. I have seen several of the mid-mowers for the 1990s B7100HSD for sale at various places including the Equipment Trader portion of
www.traderonline.com.
Hope this helps,
Kelvin