Steve. It's not the engine or me. Read the post again. Two different engines - two different tanks. This is not an ARF.
Embedding the picture in the text didn't help make the post very readable, added to the fact that the picture is too wide, causing the text to be wider than normal. To read the text, I had to scroll back and forth. Assume it would be the same for everybody.
Anyway, I recall Andrew Tinsley posting about his buddy in England with some puzzling problems after a Winter's storage. After much conjecture, a clogged muffler was the cause. So, did you use the same muffler on the two engines?
If you want to build an electric model, I think you're way better off doing that, instead of converting one, most especially a profile. But I hope it works well for you.
Steve