Rotted Exterior Wood Needs a Repair

Since you mention that the damage is structural, I’m assuming that you’re talking about the wood under the siding or trim. If this is in fact the case your options are very limited as to what you can do other than to replace, or sister new wood onto old beams or joists.
If you’re talking about trim, then you have a few more options, including stabilizing and infilling. There are plenty of wood stabilizers on the market that will firm up spongy wood and prevent further deterioration, (but they aren’t going to provide significant structural strength). For best results, you want to soak the affected wood so that the solution really soaks in, but it will probably be tough doing that with the wood still in place, especially on something like a bay window, where you’re going to have vertical elements as well. You might want to consider drilling small downward holes into parts of the wood that you can funnel the stabilizer into to increase saturation. The two part epoxy stabilizers tend to work best, but they are expensive and can be difficult to work with. Minwax makes a rotted wood stabilizer that Home Depot carries and it’s pretty easy to use, you just brush it on, then once it dries, fill in the missing/rotted section with a wood filler. Bondo is the cheapest to use for large sections, but it doesn’t expand and contract at the same rate as wood, so you tend to get seperation cracks. Exterior wood fillers work well, but again, the cost adds up quickly.
Obviously the best solution is to replace whatever needs to be replaced, but we’ve all been in situations where a quick and dirty fix is all that’s in the cards. Good luck with it.