I have recently finished fixing up 2 of these TTs and I think they are great pieces of kit IMHO.
I had the same problem as you on the 1st Rd40 as it came with a completely wrecked Ariston tonearm but not sure if it was an Opus or what model exactly, but it was well beyond repair.
I could not find a tonearm that would fit the existing Ariston armboard so I eventually replaced it with a Linn Akito tonearm which sounds superb.
The 2nd RD40 came with no tonearm at all and so I fixed a Linn Basik Plus to the 2nd RD40 that a friend of mine now has and again it makes a great combination.
I also tried both RD40`s with Rega RB250 & RB300 tonearms and again they make great sounding combinations, but my personal preference is to match an RD40 to either of the above Linn tonearms.
I made the Linn and Rega tonearm boards out of MDF and I can give you a few hints, tips and pointers about how to do this if you want to?
Or you could see if any of the other more experienced VE members would be willing to make one for you or if they know a man who can.
Sorry I cant be more help than that.