So,
@rural mouse your long-term plans could figure in here significantly, as far as how much to invest in this door fix. Though maybe you can re-use whatever you do.
I like
@RoyalChick ’s suggestion of a “found” or cosmetically damaged on-sale door as a relatively easy quick solution for now.
@bgmathteach is very experienced at using reclaimed wood from pallets and what-not and has really practical suggestions.
@rural mouse When I did 45 degree cuts for bracing without a guide or chop saw I wasn’t too OCD about it because I wasn’t aiming for super placement accuracy, unless the cut was way off which could make the brace lie too close to the perpendicular. Both ends of the brace will fit wherever the angles lay flat against the other boards. So some braces made a more acute angle than others, if not an equilateral triangle.
@Ponypoor yes deck screws are what we use here too, coated. Most of those sold now are self-tapping and I haven’t had a problem needing pre-drilling by going slowly. PS old barns here are often built into hillsides or have a built-up drive to it, for the ease of loading hay bales (unloading door on a downhill side). We used to walk out on to the beams (or climb even higher) to jump into the old hay below…