"As I understand", the MF trailers were made Humberside way by a firm call Weeks. I have one that I re-bodied. It is on 5 stud Land Rover wheels with 7.60 tyres, so I rate it at two tonnes with these tyres. I use it on my 885 (45hp approx) and was carting conventional bales this time up to six high but still damaged a trailer tyre. Most of the work was up hill with one long decline, using low gear it coped but I would not like to have been heavier.
Obviously the Fergy 35 is an earlier tractor of mid 30s hp? We used to cart up to 7 high in the 1970s with a 35 and 135, but that farm was flat and we punctured tyres.
Another limiting factor on small tractors is the tow bar, look at a few to see how badly worn they are.
Apparently one on my DBs has a 780 tow bar in good condition which is rare because they were all overloaded and damaged because the tractors would pull the load (as I was told by a DB expert).