You could try something in the 3.2 size class, like a Brison or a Fox. These should be able to swing a 24 X 8 prop. Normally they are happy with a 22X10, so a 24 X 8 should not be too much of a stretch.

I would expect about 6500-6800 rpm.

The extra weight will not be a problem.

How the plane will fly with an 8 inch pitch prop at 6500 rpm, I do not know, but I think it will be OK since this is a slow flying plane anyway. With a 100% efficient prop you would get 49 mph flight speed with an 8 inch pitch prop. Figure 85% efficiency and you end up with 42 mph.

You will probably want to reinforce the structure somewhat for the larger engine.