Here are the rules for 2008 for all senior pit bike classes, for the semrc , lcrrc as there were some confusion on other forums...

Cat A - up to 125 max cc, 26mm carb max, no flatslides, no jap cylinders/pistons/barrels/heads

cat B - up to 140 mac cc,  26mm carb max, no flatslides, no jap cylinders/pistons/barrels/heads

Modified - Chinese engine over 140cc [to a maximum of 160cc], or engine using Japanese tuning parts [not cams but anything else basically], or a carb over 26mm will have to go to Modified class.