Wow. Thanks for the info. Quick question though. Would it dramatically improve the sound of these if I added a layer of maple or other hardwood to the inside? (assuming of course that it is done in a professional manner and that I could do it for inexpensive enough to be worth the effort.)

Quick answer: No

It might change the characteristics of the tone a bit, but that's a pretty major undertaking.

My best advice to you would be to clean up the rest of the kit and sell it to a collector of vintage Japanese drums and then use the money towards getting an inexpensive modern maple import kit.

I agree with O-lugs and there is more info on this site under the Pearl section. Which these are.

We have a few Pearl collectors on the forum that love the early stuff.

Here is the Japanese Drum section on this web site which has lots of badges and other information.

David