I believe that a new one might be in order. I think that current Suburban vents even now are able to be used as a replacement. Suburban vents have changed very little over even the 40 years since your furnace has been made.
HOWEVER: if you have a 40 year old furnace in your rig, and are planning to use it, I would really recommend you replace the furnace and it will come with a new outside plate you need. After 40 years both the technology of the furnace and the safety of the original interior guts (they rust out) is questionable. The furnace takes in outside air (bottom hole) and exhausts the spent gasses (top hole) and so no interior air is used or contaminated. But if the furnace rusts out, dangerous CO gasses can come into the trailer interior. They are deadly.
I had a '71 Caravel
, which had been in Puyallup all it's life. I brought it to Idaho. I kept getting headaches any time it was cold out and the furnace was in use. To make a long story short, the furnace had rusted out and I was being poisoned by the CO leaking from the furnace. A replacement was in order and solved the problem. Please consider it very carefully.