You don't want a lot of inertial mass in the roller. The easiest solution is to go to get a roller used in manufacturing and shipping transport lines. It's about 1-1/2 inches in diameter and comes in various lengths, has bearings on each end, and spins very easily. Just bend a couple pieces of 2 x 1/4" aluminum and add a couple cross-members to hold it all nice and rigid and fit it to the bike. The roller should be wide, like a couple feet or more (three feet isn't too much). You don't want someone to come around the side of it by accident and hook it. This roller is one that you can actually bump against and not be destabilized when you're on a bike behind it.
Some people have tried mounting drums from rollers (the training kind) but you really don't need a big roller as much as one that'll spin immediately without trying to slow down your front wheel. A bigger roller simply doesn't do well.
While you're at it, re-route the exhaust pipe so it runs very close to the ground and out of the face of the riders, and if you can install a cruise control or at least a digital speedometer instead of the original dial unit, you'll be a lot more even in your speed. Most motorpacers have trouble maintaining an even speed or accelerating at a smooth rate. You can also gear the bike down a bit as well. A lot of track racers feel that a 125 cc bike doesn't have enough oomph; you do better with a 250 cc or so with somewhat lower gearing.