2.1 OHC 4. How to diagnose a loose or broken valve spring

Here's the overview. Been fine tuning my little AQ 140 with Weber carb. Went through and adjusted the valve clearances. Got them all about where I wanted them. Took the boat out the other day for a fishing trip. I ran about 10+ miles mostly cruising at 3,800- 4,000 rpms--when not idled down or running slow. Ran it up to 5,000-5,200 for maybe 5 seconds a few times to be sure I had my prop dialed in the way I wanted it-- with a load and all.
She ran great the whole trip--all the way back to the trailer.

This morning I decided to do another check of my valve clearances. Most of them were in range (cold) except one, I could easily slip a .030 gauge under the heal of the cam. It was .016 when I checked it previously.
I doubled up two gauges and could slip .040+ under the heel of the cam lobe! Then I noticed what was happening. Each time I put the gauge under it seemed like there was very little tension on the valve spring. The little cup that holds the shim just moved downward too easily.

I had some extra shims so I re-shimmed it with one about .20 mm bigger, trying to take up the slack. It made some difference but it was still way too loose. The spring (with cup and shim) still moved downward with just a little resistance.

Is this a "weak" spring? A broken spring? What's the best way to diagnose this? Frustrating. I was just doing the last little adjustment to get those valves "just right"-- then this.

Compression is great on all cylinders (165-170 psi). The motor has about 450 hours on a complete rebuild. I'm thinking of just leaving the head on, pulling the camshaft, pull that particular spring out and replace it with a used spring from one of my "parts heads". Or should I pull the head and replace all the springs now?

Suggestions?

I should have said "weak" valve spring, not "loose" in the title. I ordered up a valve spring compressor tool and plan on pulling the suspect valve spring out. Would it be advisable to buy a new set of 8 valve springs and seals for this motor? I'm thinking if one went bad, maybe the others will eventually follow. Or should I just replace the weak or broken spring with another good spring off my parts head. We used the original springs when the rebuild was done--1979 vintage.
Any advice from the resident Volvo Penta Masters?
1) All new valve springs and seals
2) or just a good used spring and seal to replace the weak or broken one?

I'm not a "Master" by any stretch of the imagination however to me it would come down to...
Am I going to keep the boat or "dump it"

You will have a better engine after replacing all the springs and seals than before. If you only do the one cylinder, you will be pretty much back where you were when you launched

Thanks for the input guys. After I thought on it for awhile, I came to the same conclusions. I ordered a set of new Volvo valve springs for the motor (8) and valve stem seals.
I converted my compression tester so I can pressurize each cylinder to hold the valves closed and in place (Valve holder). Basically a rubber hose with a fitting on each end, screws into the spark plug hole and then connected to an air compressor.
I also ordered a valve spring compressor tool so I can replace the springs and seals with the head on. Yep, I plan to keep my boat for at least a few more years. I appreciate the advice and your thoughts on this project. Sometimes when I encounter a new problem--one I never had before--it takes a while to figure the correct approach. I also think that after 36 years my original valve springs are definitely due for replacement. It's just one of those things that I would never have thought about, until it became a problem. As Kim said, if one failed chances are good another one could go any time.