'95 Plenum vacuum loss issue

I bought a '95 Montero 3.5 a few months ago. When I went to renew the tags, it failed the emissions test. From what I've learned, there is a secondary air-intake in the front of the plenum that is vaariable control. Apparently the gasket seal around the vaiable arm has worn from time and friction. The kicker is that there is no way to get to the gasket to replace it. Goodyear is telling me that Mitsu does not list the intake or any of its parts separately, and that the only way to take care of the issue is to replace the entire plenum unit! At first i was told that a new plenum would be $350. then I get a call back saying the part# was superceded and the new part # price is $850! So with labor, I'm looking at $1,200 because of a $6 gasket.

Has anyone run into this issue? Are there any ideas? I'm guessing that's the reason Mitsu redesigned the plenum, and I'm a little pissed that I have to pay an extra $500 for their poor design.

Thanks.