Take it one stage at a time. It could even be as simple as a head gasket.
Get the rocker cover off and have a look at the timing marks. If these have slipped or there is somthing obviously amiss in there (bits of metal or missing valves) then suspect that.
Take the plugs out and have a look at the top of the piston. Whilst there turn the engine over by hand (with both the plugs out) and see if it is tight or you can see anything amiss. (like the valves not moving)
All these thing could save you a lot of time / money come making a decision whether to get the lump repaired, buy another engine or break the bike and buy another.
