Rebuild or Replace
It doesn''t sound like there is anything wrong with your existing engine other than seals/gaskets which would argue strongly for a rebuild. But a good diesel mechanic could evaluate the overall condition of the engine for you (compression, heads, valves, injectors etc.)at a price that would be a lot less than a new engine and then you''d at least have facts to base your decision on.
I''m sure he could recommend & install monitoring guages on an existing engine as well. The trick is finding a good mechanic and I''m sure the board folks here can recommend one in your area. Best...GB