The first Pet .09's came out about ~1958 and had steel cylinder & fins and a smooth top aluminum head. The cylinder was held on with two screws, fore and aft.
The next iteration had four screws.
I believe the next one was the Pet-II.
The next version has a drop-in liner. All ran well in their day.
For a few bucks more you could get an OS .10 which was a notch better. I think the first Pets were ~$4.95.
Here is a pic with a Pet-II with the engine mounts cut off (before I got it)...note the Champion glow plug in the upper left. Upper right is an OS .10. The three on the bottom are the final versions with the drop-in liner. They are baffled engines but you must remove the head to see it because they have a step-down baffle.
If you are looking for a kit plane, a Flying Clown from Brodak or a SIG Akromaster (if you can get one) would do nicely. Fly on 42' x .012 lines and use 15%-25% nitro fuel. As someone mentioned, if you use RC fuel add enough castor to have 20%-22% lube. Since it has already been run I would suggest all-castor lube.

Good luck.

George