You could probably fabricate one pretty cheaply with heavy wall RHS tubing. I assume the dealer wants too much for one? if they are even still available.
What type of attachments are you looking for? Any attachment small enough to fit, you could make an attach plate for to suit the machine.
I just did a quick search and found a few members with this same machine. You could try doing a search for 371 in the forum, contact some of the members that posted about their 371 machine, they may be able to give you measurements so you know what is involved. If you aren't up for the task of making one, the details could be given to a fabrication shop and they could build it?
I know it may not cover workplace health and safety, but i figure, any ROPS is better than nothing at all.
Its a rather old machine, i wish you luck in your search.
Is there a good power supply for the 7 pin plug