The chassis is one made by B&B who were bought out by ALKO and basically shut down many years ago. If the chassis still has the original hitch it will be an imperial sized one, I cannot remember the exact size they were. The importance of that is that you do need a matching towball to tow if possible, as the hitch will be slightly larger than current ones. The current metric towballs do fit, but the correct size is needed, as if the hitch gets very worn it can result in the trailer coming adrift, and the age of the chassis would suggest this is possible. Probably better to replace the hitch in practice.
I have dealt with a case where this has happened with a trailer carrying a boat.
If you need a new jockey wheel, it is possibly best to take it off and visit dealers who might have one. I suspect boat accessory dealers might be a better bet as they will be more used to this, as boat trailers tend to stay in use longer.