The power designed for the ringmaster was an old Fox 35. It's your basic 9 ounce, anemic stick spinner. Your target engines produce about twice the power. You could probably pick up a used Fox 35 or 29 on Ebay for as little as 20 bucks is my cheapest, never go over 30. Generally they are just broken in or run fairly well. You can also send a engine of any kind, Broken, torn or worn out, to Fox Manufacturing and get a brand new Fox 35 for half their listed price (bout 50 bucks) OR you could buy an OS 25 LA CL version new or off the bay and that would be equivalent power, cheaper fuel than the Fox (it'd need castor fuel) and it wouldn't vibrate near as much.

I'd probably go with the LA 25.