Just saw this so you may have already worked out a plan.
One method I have used successfully is to remove the two studs from the fly wheel and replace the clutch shoes with a metal plate of the same diameter and near the same thickness as the retracted shoes. Holes are bored for the two studs and a threaded hole precisely machined in the exact center of the plate is made to receive the threaded shaft used to hold the drive rollers. A method of supporting the shaft with bearings, preferably inboard and out, is highly recommended to save stress on the engine crank shaft.
I have not used this method with a GX31 but have with both Robin and Mitsu engines.
HTH