Good afternoon! I have a new-to-me 1991 300d with 392,000mi on the clock. She was neglected by previous owner and I'm trying to find and address issues to get her roadworthy again. I'm close, but have hit a snag and need guidance surrounding the ALDA system. I have some fairly specific questions, all help appreciated!

Following an unrelated cooling system repair, I was driving her quite hard to ensure cooling system functioning properly. During the test drive, I was at 100% throttle in top gear up a long ~5% grade running ~80mph (so full boost) when there was a sudden and discernable loss of power that has persisted in all driving since. This is not fuel filters as 1.) they're pretty new and 2.) power loss was instantaneous vs the slow fade of a fuel filter.

Prior to this issue, I believe that entire boost system working properly as power was fantastic and exhaust smoke was non-existent. Now, while I still have zero exhaust smoke, but power is poor at times I should be experiencing turbo boost (unboosted power off idle is unchanged).

My hunch is that I popped an ALDA capsule. I welcome other ideas, but my questions center around this hunch:
1.) If a capsule popped is this repairable, or does the whole valve have to be replaced?
2.) If I need a new valve, where the heck do I buy one...I can't find any online!
3.) My intake needs to come off for other reason (cleaning out EGR gunk and to gain access to glowplugs for replacement and pre-chamber reaming). As long as I have it off, this would be an optimal time to replace/service/repair the ALDA. I'd love to avoid unnecessary knuckle-busting; is there a way to rough-in ALDA adjustment while I have the intake off?

Thanks ahead of time for your thoughts and expertise!
Brad