In case anyone is still interested in the "Happy Songs"
"album" by "1910 Fruitgum Co." (there's a reason for all
those quotes), I've found that at least 6 of the 10 songs on
it are available on CD, five of them in identical versions as
far as I can tell. The sixth, "Bingo", is available in a different
version as "Bingo, Bingo" on the Super K Kollection Volume 2.

So here's a list of where you can find the other five tracks
from "Happy Songs" that I found on CD:

1) The Clock - The Super K Kollection Vol. 1, Track 8
2) The Sweetness of Soul - The Super K Kollection Vol. 1,
Track 11. The artist on the CD is listed as Fat Man's Music
Festival, but it is the same recording on the "Happy Songs"
album where it is credited to 1910 Fruitgum Co.
3) Go Out In The Rain - The Super K Collection Vol. 2, Track 4
4) Candy Cane - The Ohio Express - The Super K Kollection,
Track 4. Again, the exact same recording. Attributed to the
Ohio Express on a CD collection of their Super K years, but
the "Happy Songs" LP, which certainly precedes the CD
timewise, is supposedly an album entirely by the 1910
Fruitgum Co..
5) Feelin' It - The Super K Kollection, Vol. 2, Track 14.
The Super K Kollection CD lists the artist of "Feelin' It"
as Crossfire, but it is the same recording as the one used
on the Happy Songs album by 1910 Fruitgum Co.

I would venture a guess that the songs credited to other
artists on CD were indeed by other artists, not the 1910
Fruitgum Co., which oddly enough is one of the few bubblegum
groups which had some cred as to actually existing. So this
shoddy "album" doesn't do them any good for any kind of
credibility, which is probably why it didn't come out here.

But if you're like Doug and that tape hiss drives you crazy,
at least half of the album is available on CD, albeit
credited to the groups that most likely really did the songs.

Finally tracked down an answer on "Mary Wanna": according to a post from the man himself on an old Spectropop page, the song was penned by Austin Roberts, but K&K pulled it from the album after finding out it'd already been copyrighted under a different publisher. Still curious about where "Go Out In The Rain" came from.