The original tyres were 6.00 x 16 Goodyear Multirib, which have a 5 rib smooth pattern tread (the tread cut outs are not in a zig-zag pattern). These are hard to come by but are sometimes available, generally priced at around £80 each. Telephone first to check on availability.
Massey Ferguson tractor front tyres (6.00 x 16) will fit and do a perfectly satisfactory job but they are of a 3 rib pattern and are not authentic.
Other sizes may be available with the same tread pattern, as fitted to some trailed implements, primarily in sizes: 5.50 x 16 (replaces 5.00 x 16) and 6.50 x 16. Again, telephone first regarding availability.