Put fresh premium fuel in it double dosed with Sta-Bil and put some oil in the cylinders.

Why an engine rebuild? Does it burn oil? M104's run very well for a very long time, mine had 361K miles until a head gasket failure. Compression test at 180K and around 310K were perfect. Consider a compression or leak down test before pulling the engine apart. The rebuild could be $$$ and time with no gain.

Good luck!!!

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer