Availability on new boards will most likely be difficult to impossible. First thing would be trying to get your hands on schematics for them, which will also be difficult, IMO. Depending on the failures, you could get lucky with some straightforward component replacements to very unlucky with torched/destroyed boards. For my money, I would stick with either new with warranty or used with warranty.
Sometimes Free isn't cheap enough.
Hard to get an estimate on repairing electronics because by the time you troubleshoot the problem you've already done 90% of the work. There is probably a good reason someone hasn't repaired it already. Probably did a once over and decided they didn't want to invest any more time in it. Time is money.
repairing a switching amp is a hard (if not impossible) thing to do.
unless you have access to equipment to actually measure the waveforms then don't even try.
even if you do, there's so much going on with so many different design that make it almost impossible.
gone are the days where you could spot the problem by visual inspection and validate the problematic component with a cheap multimeter.
it wouldn't be worth the shipping to send to a service center. These are manufactured so cheap (may under $50) these days. Your best bet is to look for a used unit and not pay sales tax and be done with it.