Considering the performance I've been getting with a Dr1 at 9 lbs... I think you could use a O.S. .91 FS and get better performance than you have stated... I'd bet you started out using a .60. I'd also bet that you are using a propellor which is more appropriate to a single wing sport plane.

If you simply switch to the longest, lowest pitch prop your current engine would turn... you'd probably be much happier. Draggy planes (biplanes by definition are draggy) need better low speed "authority" from the prop. Getting more disk area and lower pitch gives better pull for take-off and climb.

The O.S. .91 FS on my tripe has the power for unlimited vertical... with smoke sytem active. (which drasticly cuts power) But... I'm using a long, low pitch prop. I feel that the engine is under-propped right now... and going above 75% throttle with the smoke off gives no improvement in top speed. With the O.S. recommended 12X10... I doubt it would fly worth beans. I've been using a Bolly Clubman 13.5 X 6, and I'm going to put on a MA 15 X 6 "Classic" (which is a black nylon with rounded tips) to try for torque rolls. (much heavier prop for more torque effect, and more prop blade area.. bigger disc...) If they made a 16 X 4... I'd try it.