The restrictor plate goes under the stem that goes up inside the oil filter. I've never had this problem myself, but have heard from others that have. All you need to do is spin the oil filter off, and then spin the stem out and there should be a restrictor disc under there. You can see it on page 2-5 of the Oliver service manual, figure 2-5. The filter is a by-pass filter, so without the restriction more oil ends up going through the filter, lowering the pressure. This was more of a problem on the units with the aluminum filter base.

On the front seal, it might not dry up, but it is worth a try rather than tearing it down again. If the cork seal is ok, then you're going to have to do something with the cover to get it smoother.