The only small engines without drain plugs I've come across so far have been on push mowers which can be completely drained of oil and any sludge via the filler by tipping them over.
I presume any motors without drain plugs have been designed to allow complete drainage via a vacuum tube system. The Honda GXV390 doesn't look like it would be possible to remove all the oil by a tube through the filler.
On machines such as generators and quads, 4WDs, etc.where it is difficult to allow oil the be drained from the plug without making a mess I use old aluminium foil baking trays moulded to shape, similar to Norm's suggestion.
