No simple answer to this one.
To dig the remains of rusted bolts out is likely to be somewhat idiosyncratic to each hole; the one thing that you'll need around all of them is clearance.
If you're not removing the
engine, it may be possible to remove each bracket from the
engine sequentially and do what you have to do to get the old bolts out. A
wood chisel or die grinder may
work to expose a section of the bolt to which you can attach a pair of vise grips.
Once you get them out, if the
wood isn't rotten or wet, you may be able to just use a longer or oversize (or both) bolt; if this is the case, this is one application where 5200 makes sense,
If the hole is too wallowed out, you'll have to fill it with something;
epoxy and milled fibers is a good choice, since it won't
rot, but a lot of people hole saw the hole to a standard size and
epoxy in an appropriately-sized wooden dowel.
If you can't get the old bolts out, sometimes you can just rotate the mount a few degrees to screw into new material; this is strictly limited by application, some mounts are designed specifically to be mounted fore and aft.
Probably the gold standard for lag bolts in this application would be silicon
bronze, but at 7.00 or so apiece for 3/8 x 2 1/2, 316 SS might be almost as good. Plenty of people just use hot-dip galvanized and keep an eye on them.
The problem with both 316 and galvanized is that the heads can appear good, and the business end corroded to worthlessness; good silicon
bronze doesn't suffer from this drawback, hence 60.00 or so might make more economic sense in the long run...