Wow AC- you've got some neat equipment. A real good source for comparison, is to go into an aftermarket catalog like OEP, Stens or Rotory. There you can get dimensions of blades and hubs and compare it to what you've got. If you then would like to use the genuine MTD goods, that number will be listed as well.

A very intelligent question.
Risky to match the existing blade as it is likely that its the wrong one.
And that is something - knowing the year of manufacture.

It had a 3.5hp Tecumseh. I just repowered it with a low hour 1979 B&S 3.5hp. I used a blade adapter off of an older Murray and bought 3-n-1 combo blade. It works great