LPG regulator problem- 25mph max?

I have an LPG conversion on a Ford van that is ~30 years old (yes, it's ancient, tho with sentimental value). I'm able to handle most problems like replace fuel line locks and even a rebuild on the Impco carb unit that sits atop the gas carb.
The current problem is some hard limit on fuel supply, such that the engine will start and idle smoothly and rev up to ~2600 rpm/25mph max, but then runs very rough (overcarbureted?). I can only assume the issue is the regulator. Is there a regulator rebuild kit available? Replace or repair?
BTW, the regulator was installed in a very awkward place, so it is difficult to inspect or service.
The van has been used very little recently. It sat idle for perhaps a year while I was away. Some deterioration was expected, but this problem stumps me.
Any advice or referrals would be very much appreciated.

The regulator has a screened air hole to allow the pressure regulator to work. If it has been sitting there's a good possibility that some bug has used the area for a nest and has blocked the flow of air. I had it happen to me when we had a ice storm, it froze over and shut the gas down real low.

Since you have a Impco carb. You most likely have a Impco convertor.
Get the model number off the convertor and you can buy a rebuild kit.
They are not hard to rebuild I rebuild them often. Just be sure and follow directions.
Some people call it a regulator, proper name is convertor.