We would have to hear the knock to be reasonably sure, but the usual cause of an engine knock on an engine that has not been dismantled in a long time, is a worn big-end bearing. Big end knocks do tend to be louder under load, though you can usually hear them at any load. In general, big end bearings only fail on correctly loaded and correctly assembled engines if there is a failure of lubrication. That, in turn, usually happens because the oil level is low, or the machine is run on an excessive slope, or a combination of the two. Once slack develops in the bearing, it will usually get worse fairly quickly if the engine is run under load. Repair is usually possible, requiring at a minimum, replacing the connecting rod, but usually there is damage to the crankshaft as well. If you choose to repair the engine before the connecting rod breaks, it is usually not especially difficult, but if you continue to run it under load the rod will probably break, and it will then be doubtful whether the engine is economically repairable.
The owner manual for your engine is available here:
http://engines.honda.com/pdf/manuals/3189120.pdfUnfortunately your engine is one of a relatively small number of Honda engines for which no workshop manual seems to be available on-line.
