"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.