Replacing a top end should be done when compression becomes weak and engine performance is not up to par - itís a way to freshen up an engine that has a lot of time on it.

When repairing the top end of an engine, the case should be pressure checked for leaks.
If there is a leak in the case or through the crank end seals, you will have a lean condition (too much air) that can damage the top end.
Go through the bottom end to ensure it can handle a new top end without destroying it. Everyone would prefer an easier/less expensive fix, but if you take short cuts on your repair your time and money may be wasted.

Using a top end kit is not a fix for zero compression in a cylinder or severe scoring to a cylinder. If you have severe damage to a top end, you should replace the entire engine or use a cylinder exchange kit and go through the bottom end as stated.