If you need a modded oil cap with a hose fitting for testing purposes, a rubber stopper can be modified to work.
The one below was modified to fit inside a 1998 Sportster oil tank (hole is tapered, app. 1.274“ to app. 1.224”).

A rubber stopper can be purchased at local hardware stores. The one below was bought from Lowes.
The Hillman 4.6“ black neoprene hole plug (taper 1-3/8” x 1-3/16“) will work by simply drilling a hole through the center of it.
However, if it can't be found, the Hillman 1-5/8” x 1-5/16“ x 1” can be slimmed down to work.
It is actually too big and the smallest part of the taper is all you can get into the oil cap hole.

The 1-5/8“ plug was used below. See the before and after pic below. The plug will go in the hole but not very far.
It is possible to simply hold it into the tank since it will go. But the one below was shaved to allow hands free operation.

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