Not a good bet. I built one last year, and the firewall was as square as can be.

NGH00d, I don't think the angle is all that critical myself. There are many other things in the plane, like a little wing warp, or side-to-side balance, or the ailerons not being perfectly set up that will affect the flight to a greater degree than the engine side thrust.

Still, if you want to obsess on this, it's not too difficult. Get a protractor, and draw an actual sized picture of your engine as seen from above on its mount. Then draw a line down the centerline of the engine, and another line starting where the centerline hits the firewall, and pointing 2 degrees to the right. You'll see that 2 degrees is only about 1/16" off from straight at the end of the prop bolt (maybe less). Now, draw a line exactly perpendicular to the 2 degree line, intersecting it where it hits the firewall. That will tell you how thick a washer to put under the mount on one side. It'll be about 1/32" or so. When you screw the mount to the firewall, just put one washer between the mount and the firewall on the right side.

Myself, I use through bolts to mount the engine to the mount, so I just shifted the engine to the right a little as I tightened down the screws that hold the engine to the mount.