I don't know all the differences, but I know the mod motor had extra boost ports in the cylinders that the normal one didn't. I think they had twin HD carbs as well. My brother would know everything that was different. He is into the old cats big time and has one of the mod 634 Pumas. His also has the "fish" pipes on it but I don't know if they came that way from the factory.
I can ask him all the differences and get back to you if you want. He pretty much runs the Arctic Cat club of America web site and really knows the old cats.
"The MOD had the booster ports with the pistons drilled to match up with the booster ports. The intake port was bigger and the exhaust port was bigger and raised a little bit. The heads were shaved a little bit. The timing was set to the high side of the stock specs range. It used twin HD Tillotsons and the "fish" pipes are factory (the 440 MOD (Kawasaki) also used fish pipes and the 340 MOD (Hirth single cylinder) had a normal, round expansion chamber). The chassis is the same on both standard and MOD sleds. The 634/634 MOD used the "big mouth" hood, as did the 340 MOD. The 440 MOD used the "axial flow" hood."
This is what my brother sent me about the mod Puma's, Like I said he runs the ACCA web site and has tons of old Cat info.
I have driven the 440 MOD '71 that we sold new back in the day. Current owner contacted me about its history and wanted some shots of me on the sled, since I still have a couple jackets with our old team patch on the back. A real retro rocket from 0-60. The one change he made was the swap to 1971 EXT crossover chamber pipes. More pop and also the outlets are down in the snow, not in your face like the fish pipes. He got tired of that racket in the face years ago!