Cost involved in this "repair" sound prohibitive. I see your second hand MS309W and raise you by Rel Quake 😃Source a second hand MS309W??
An older NAD or Rotel power amp that can be bridged springs to mind.
How much power do you want/need.
Could always look for a PA amp from a company like crown.
How much are you prepared to spend? According to eBay sold items a complete MS309W sold 3 weeks ago for £39.99 plus delivery. Someone certainly got a bargain but average selling is probably is sub £100.It's maximum output is 250w, so ideally something similar to that.