We're not familiar with the mitaka brand so I can't speak to the quality of their products. Using second hand parts is almost always problematic. The problem with an engine with catastrophic engine damage there are often parts damaged that are difficult, if not impossible, to see with the naked eye. Many parts are heat treated and often engine damage is a result of oil deprivation which also means the engine experienced extreme heat during failure. This extreme heat can reduce the heat treatment of these parts and they can subsequently fail. It's also not uncommon for used valves to fail if reused under these conditions. Springs may no longer offer the correct spring tension or they can outright fail. Valve stems, cotters and retainers wear as a unit. If a valve is installed in a different used retainer with different used cotters they can fail causing a valve to drop. So, the best way to ensure there won't be continued engine failures is to replace all affected parts with new OEM or quality aftermarket parts.