That was my first thought too. The rules are written pretty narrow and I would be surprised if there were lots of tractors in that type of class. I can't see policing horsepower very effectively either - someone always seems to be disappointed.
My thoughts are along the lines of what you describe - if they want at bone stock class, make it slower than 3mph and maybe lengthen the sled chain. If they want really stock then make it 2.5 mph plow speed at factory rated rpm. I'm all for everyone having fun.
On another note, the 4mph and 6mph modified USAP classes are really heating up around here, especially with surrounding clubs. These are my favorite classes to pull in. My hope was to draw in pullers from other clubs to boost hook numbers using USAP rules. I would like to see our club's rules appeal to a broader audience.
Thanks for your responses! I wanted to bounce this off the YT community before I go to the rules meeting this weekend.