In fact the engine continues to be hard to start. I spoke with a mechanic who suggested a valve adjustment and a look at the flywheel to see if the key had sheared. I looked at the flywheel but did not pull it and adjusted the valves, which were out by 0.005 inch. The engine certainly has more power now, but is still hard to start. However I think I may have stumbled on a cause. The engine has an automatic compression release. I think the spring is broken, which would explain everything. I'll follow up when it's fixed.