Tires look good, by the green algae if it cant be fixed it could be used to show a north heading. There are companies that make aftermarket parts for the old Kubota's and I believe they share some parts with the ISEKI or Satoh tractors, the tough part is cross referencing part numbers, because even if they were the same part from the manufacturer, they were different numbers on each brands parts list. There might be a list of the part vendors for the older Kubota's. These older Kubota's were made in large enough numbers to justify aftermarket vendors. This link is good because while they might not have every part they do show pictures of the part that might help in sourcing one from another manufacturer or vendor.
http://www.yesterdaystractors.com/cgi-bin/store/compact_model_parts.cgi?SearchArea=Kubota&md=B7100