I found a service that will reproduce decals for the Hydra center caps.

For an 8" x 13" waterslide sheet that would contain 8 decals:

One time set up fee 15.00
Printing 32.00
Shipping 9.00 Total $56.00

Allrounderco posted the part number (1343663) for the Hydra caps on this thread and the caps are still available from Tasca for $12.14 (plus shipping and the cost of wire springs).

So I don't plan to pursue the decal route. When I paint the wheels I will just order new caps and springs. The original caps can be sold to offset the price of the new ones.
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Any twenty minute job is just a broken bolt away from a three day ordeal

When new, I think that Hydras have a clear coat as well.

I painted one set of wheel covers (flat, dished covera with 22 holes) maroon to match the interior of our '88 745 GLE - our high school colors are maroon and grey like the car. My daughter drove the car when she was in HS.

For an insignia, I made a Volvo V 2 inches high and cut my decals out of silver reflective tape. When done, I saw that the center cut out of the Volvo V was a nice arrow.

I installed the arrow above the Vs. I even made the arrows face the direction of rotation!

Starting with a white or dark first coat on the wheels can help lighten or darken the final top coat to better match the plastic caps as long as you don't make the top coat too thick and apply it fairly evenly. Beyond that it would either be living with a mismatched or totally plain cover (both yecch), pinstripe masking of the black decal (PITA, destined to fail) or having custom new decals made up ($$). I'd spend extra time trying to custom match the existing wheel paint. I expect a good auto paint supply shop should be able to color match it without a paint code.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now