I was curious about the O-200, so I ordered the FAA CD for PA-15 15-396. (I tried uploading these as attachments to the site, but they were too large.) I put them on my web server for those who would like to peruse them:
http://squeezehead.com/flying/15-369/
If you don't want to wait for the download, here's the short story.
In 1971, the A&P owner installed an O-200A into his PA-15. He wrote the following letter:
12-15-71
FAA:
I request that my Piper PA-15 N4663H be certificated in the Experimental Category to show compliance with the FARs following the installation of a Continental O-200A engine. The flight tests will be conducted within a 25 mile radius of East Colfax Airport. I would like the certificate to be issued for twelve months. There has been no external changes [sic] to the aircraft.
The 337 form lists the following description of the mods:
Aircraft fitted with Continental O-200A engine without starter and generator. Installation done I/A/W Piper Powerplant Drawing #11828 with the following exceptions:
a) Cessna exhaust assemblies P/N's: 0450400-21 & 0450400-25
b) Carburator [sic] air box P/N A40793.
c) Engine cooling baffles P/N's: 4-882-R, 4-881-R, 4-879, 4-880, 3-629, 3-360, 1-3703, 1-3704-1, 1-3704-2, 1-1737, the preceeding [sic] are Champion P/N's.
d) McCauley 1B90 CM 7144 Propeller.
e) Cowling modified from Piper PA-16 to fit aircraft.
5 pounds of lead ballast are required to bring the C.G. into the C.G. Range. Ballast located on tail wheel attach points and is +165.0 aft of the Datum.
This installation is for the purpose of evaluation for approval of a S.T.C. and must be operated in the experimental catagory. [sic]
The following placard required: Fuel Tank must be 3/4 full for take-off.
Experimental airworthiness certificates were issued in 1972 and 1978.
The Internet tells me that the A&P owner died in 1988 or 1989. At that time, the plane was registered by his wife. She died in 1993. I called around to the local airports and found someone who knows something about the plane and its history, and am waiting to hear back. I'll let you know what I find out.