The bolts are probably 5/16. Do a google search on ASME (American Society of Mechanical Engineers) torque specs and in the chart find your size, material and condition of the medium...corroded, threads chased, lubricated.
On getting your existing bolts out without breakage, impact is the only way to go. If you don't have a wrench use a regular boxed end wrench and a hammer. If you do have one put it on a low pressure setting.
Dose the bolts down with PB Blaster or better penetrating oil the night before you do the work. Probably won't do you any good as the paint probably is sealing off the bolt to the plate interface, but try it anyway.
Get on the bolt with your wrench holding in place with pressure on it with one hand and using the other with something like a 10oz ball peen hammer, start smaking the blot. As soon as you get it to break loose go to the next one till you have cracked them all.
Soak down with PB over night and the next day come back and try to get a half turn out, working your way around.
One more time, soak and come back the next day and you should be able to get them all out without damage.
When out, clean out the threads of the bolts with a power wire brush and run a 5/16 fine (as I recall...check the threads on the bolts) blind tap with more penetrating or thread cutting oil on the tap down each hole to clean out the threads. Blow the holes out with compressed air.
Upon installing, use Blue Thread Locker on the bolt threads before inserting. This will qualify them as wet threads when looking up the proper torque.
Personally, I'd forget the torque wrench and just run them all in snug then using an X pattern, like any plate be it an engine head or whatever, work your self from the center to the outside tightening as you go, 3 times. The last time would be after about an hour, giving the gasket time to settle in and back with the impact, smacking moderately until the bolt stops turning, then one more smack.
On the gasket, put it on wet, and wet means with Blue RTV silicone. The manual usually talks about dry, but after 50 years of fooling with engines, marine included, what is written in the book and real life are 2 different things. When the engine is built there is no corrosion and everything is new. After 20 years it's a whole new world and the RTV ensures you will not have to come back and won't break your bolts or strip your threads getting your cover back on without leaking.
That's how I'd do it. Others probably have other opinions.
Mark
