Hi Bella Sogno,
I just removed and rebedded my 1991 P42 side windows a couple months ago. I have always used Dow Corning 795, but I don't see why Sikaflex wouldn't be just as good. I think you could go either way successfully.
The side windows are just plexiglas, and they are flat. They are mostly straight-edge ninety-degree cuts around the perimeter; however, there are a couple of corners that will need to be shaved with a router. I would second Stuart's suggestion that you take them to a glass shop and have them duplicated.
I didn't have the time to do that, so I just rebedded my old crazed windows. My boat is 26 years old, and the original Dow 795 had leaked for some time. You can generally buy yourself a couple of seasons by cutting out the old sealant around the edges and squirting in new sealant into those spaces. But the real answer is to rebed (with or without replacing the windows). I wish I had the time to have new windows cut, but that is just the way it goes.
I'm not the fastest guy in the world, and I like to work with a beer in one hand, so my time estimates tend to go long. It pretty much took me most of a day to remove each side window, clean it up, tape it off (inside and out), and rebed. Had I had the time, I think I would have made an appointment with a window shop to bring in all windows at once and have them cut new ones on the spot. It'd only take them a couple hours or so for a pair. (I also had a new forward hatch lens made, and it was only 1/2 hour of labor, with the new 3/8" x 2' x 2' lens costing about $130 total).
Cheers,
Paul