Generally, the greatest demand on the drive train of a reel mower, is when mowing up a steep slope or powering it up a ramp onto a trailer or terrace. I agree that if you only limit your ambitions for climbing ramps and kerbs to whatever stalls the engine, the 6.5 hp engine could be a problem.
In normal mowing the engine weight and the rotating inertia are important, but within reason, extra power in itself should do no harm. The drivetrain can only be harmed by too much engine power if the mower meets resistance and therefore the drivetrain has to transmit all of that power. If you pick up a bolt between reel and bedknife for example, the engine will be stalled immediately, more or less regardless of its power, since the governor will not even have time to open the throttle before all rotation ceases. The factor which will cause damage in that situation is the momentum of the rotating parts. Hence in routinely mowing flat lawn, the main thing to avoid is an engine with a cast iron flywheel, rather than one with a moderate amount of excess power.
