OK, back to basics.
If you look at the picture showing the view from in back of the engine, it looks like the prop is located below the bunks that are supporting the 'toons. That means the engine is sitting plenty far down, and the potential for venting at low speed should be pretty much impossible. As well, I don't see anything in front of the prop that might screw up the water flow to the prop. So.......
Back to potential for possible slipping somewhere in the drive train. You might try standing on the prop blade with the engine in gear. Check while in forward and reverse. If it slips, try standing on the other side of the prop. One side should hold in both forward and reverse. If it won't go 5 mph, if the prop is bad, or linkage not working for whatever reason, something should show up in that test. If it checks OK, then something else is going on that's not allowing that prop a proper bite on the water.
Is there any chance the old engine uses a different prop shaft, and we have the wrong prop on this new engine? Have you discussed this problem with the selling dealer?