I really think the engine mount arrangement was designed around using a Saito engine for power. with my 125 installed the upper throttle hole is in perfect alignment. I think they tilt the mounts to aid in getting the stock muffler further under the fusalage and to get the head of the engine away from the top of the cowl. It also sets up the carb inlet close to center on the fuel tank with this arrangement. Using the OS engine you most likely will have to do a little fabrication to get it to work.

I am thinking your .91 will fly it decently but it might be a little under powered. even with the Saito 125 in mine I wish it had more.

Here is a picture of how mine looks.