Mike,
    I have a HP-70 also and it has the very same carburator. I was able to get the parts through the local AGCO dealer after a few tries(new kid at the parts counter). From what I count, there were at least 10 different carbs. used on the 70 series tractor. I wouldn't stress too bad if I had to replace the carb.... parts wear out, kinda like a tire. If you do replace the carburator, label the old one and put it on the shelf so that in 10 years you will remember what it is but you can make the original whole again.This way, you will be able to use the tractor.
   I think it's really great that you have  an old family Oliver... that makes it extra special! <!-- s8) --><img src="{SMILIES_PATH}/icon_cool.gif" alt="8)" title="Cool" /><!-- s8) --> 
         Cal