Hi Carlos,
Thanks for the update on my old house. I sold it in 1978 and moved to Del City. I left OK in 1984.
The size numbers a real deep into the fins on the cylinder. Since you have a bridge, it is safe to say that you have two 25cc engines. The plastic back plate is fine after triming and sanding the back flat. I removed 100 thousands (about 3/32") from the back of the case to reduce its volume. Don't remove the bridge, or your ring will catch in the port and break.

Later, Dave