That engine is a Merco .61. It is an excellent engine as far as I am concerned. I have a Merco .35, a .49 and a .61 that I bought new in the early 1960s That were used heavily during the 60s and early 70s until I moved to where there was not much active modeling. I retired a few years ago and moved to a farm in central Wisconsin and started building and flying models again as I now have my own model airport and lots of room. I dug out my old RC Models and found my old Aeromaster Biplane with the .61 still in it. The plane is only good for hanging in the barn but even though the Merco .61 was seized up with congealed castor oil and other gunk I removed it and soaked it in Lacquer thinner and then transmission fluid and dismantled it and it cleaned up good as new. I am flying it now in a Thunder Tiger Trainer and building an old Kwik-Fli 2 kit that I have had for years for the next plane. It runs great and really pulls the trainer around. I fly it here on the farm without a muffler as it was meant to be run but I have a muffler I bought back in the 70s and it fits ok to so if I ever fly at a club site or in town I can put it on.
That Merco should be a great old engine but you must break it in slow and rich, it gets very hot if you lean it out to soon. Break it in easy and it will run for 30 years.
Dave Carr Wisconsin P. S. My engines are older then yours and have only one plug. The twin plug came along later.