If you just want to measure the gap between the liner and the impeller, just remove the jet foot with the grates in it.
Then use a feeler gage between the edges of the impeller and the liner to see how much clearance there is, if there is too much clearance remove the impeller and move it up by removing a washer from the top and placing it on the bottom of the shaft, assemble and check clearance. I don't know if the washers are different thicknesses, might have to play with it by switching washers around until it's right.
Can't help if you need a new replacement liner, haven't had to do that yet.
Might try GOOGLING> how to replace a liner in a xxxxx outboard jet if you don't get any replies on here.