G'day Kye,
The use of the old spark plug to replace the decompressor is a 'bodge' technique to get a mower running after the decompressor unit has failed, as the deco mounting hole has the same thread. I doubt that the 'previous tenant' cut the hose; probably just pulled it off the connector.
Should have plugged the free end of the hose too; the mower will run lean if it wasn't.
Part of the reason that Victa went the decompressor valve route, was that most of the Zip recoil starter parts [and dress cowls] could then be made cheaply of plastic, rather than the all-metal construction of the older recoil and impulse starters. As CyberJack says, the Zip starters aren't designed to handle a motor with full compression.
The 'long type' deco valve for this engine is no longer available new, so you'd have to find a rebuildable one [with the 3 spring clips around the diaphragm chamber] from another engine. Rebuild kits are available from the ODK Shop.
