From the "Sounds and stories of the Justice League of America" LP (yep, this song does exists! I tried to type out the lyrics by ear, because there isn't any an already made text for this).

LYRICS

Plastic Man!
Plastic Man!
Plastic Man!

He's a real good guy in a bright red suit
and he always wears goggles and a belt; that's a beaut'.
He was a crook, but he reformed and then
he reformed and reformed again...

Plastic Man, Plastic Man
The one, the original elastic man
Always in great shape for the shape he's in
The fantastic...

Plastic Man!
Plastic Man!
Plastic Man!

He can stretch his neck tree top high
and stretch his arms 'till they touch the sky
and stretch his legs 'till he's so tall
he could bounce the Moon like a basketball...

Plastic Man, Plastic Man
The one, the original elastic man
Always in great shape for the shape he's in
The fantastic...

Plastic Man!
Plastic Man!
Plastic Man!

*instrumental*

He's a quick change artist beyond compare
No one else like him, yeah anywhere,
and he's one guy the villians just can't catch
You can count on him to win in the stretch...

Plastic Man, Plastic Man
The one, the original elastic man
Always in great shape for the shape he's in
The fantastic...

Plastic Man!
Plastic Man!
Plastic Man!