Alex,
The idler/oil pump drive gear is mounted on a spindle in the form of a hexagon headed plug with a shaft projecting from the top. This shaft should be long enough to locate in a matching hole in the timing cover. The gear then has a ballrace for the inner bearing and a small flanged bronze bush in the worm drive end. The only plate behind the idler gear is the locking plate (and screw) and does not affect the spacing of the gear from the crankcase.
While you have the timing cover off, check that the locating pin is engaging properly with the oilpump shaft. This engages with an angled groove that gives the shaft a reciprocating action as it revolves. The pin screws into an extension of the pump body and has a thin locknut.
Also, check that the release valves are not stuck up - there is one in the pump body (that allows excess oil to recirculate) and if your engine is the same as the late dirt track engines, there is another valve in the form of a banjo bolt just above the quill feed. This is a legacy from the days when a hand pump was fitted and it allowed oil from the hand pump to bypass the mechanical pump. If jammed open it will allow oil to return to the feed side of the pump instead of flowing through the sight glass.
Regards,
Eddie.