Grab up one of the harbor freight compression testers and check the compression. Most accurate way to find out if an overhaul is in your future. Do this after the valve adjust though since tight valves will give a false reading.

For a quick test, plug in the bock heater. Let it sit for a few hours to get nice and warm. Then try to start it. If it starts when warm like that you really want to get that compression test done! An old 300D of mine went south on me and for about a month the only way I could get it going was the block heater. Compression results on that motor were sad to say the least!