I purchased a used Midwest Super Stearman, with Saito 1.50 on it, this summer. If flies beautiful, scale-like aerobatics. The 1.50 is more power than needed, but not heavy enough to offset the tail weight. I have 10 oz of lead bolted to the motor mount, and a 1200 milliamp receiver battery against the firewall, under the fuel tank.

I am not informed about gasoline engines, but as steve mentioned you'll need the weight anyway. During your building keep the tail end as light as you can without sacrificing strength or structural integrity.