The low compression is the likely problem. You can buy a piston for a D-series engine on eBay but they're a bit pricey, IMO. And that alone might not be enough.
If I were you I'd keep the mower and keep my eyes open for deals on other LBs with D-series engines. Then you can cross parts. I've made several good mowers that way.
Why throw it out? It has lots of good parts. If you don't have the space to keep it you could take it apart and sell the parts on eBay (I'll bet you could do pretty well but it does take time.)
I imagine you're probably frustrated about this but throwing it away is such a waste. :confused2: