OK, let's hope the cylinder head sealed properly when you bolted it back on, and there is no problem with compression. If that turns out to be correct, the oil-in-the-plughole test has shown that the piston rings are in good condition. Next step is to service the engine. First, check the amount and condition of oil in the sump.

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This diagram showns you how to check the oil. Follow arrow 2. The plastic plug with two bumps sticking up, is the oil filler plug. Put the end of a screwdriver across the two bumps to unscrew the plug. As dotted line 3 shows, the oil should be full right up to the top of the thread, so if you put any more in, it will overflow. The oil should be yellow-brown and slightly transparent, not black. Please post a picture to show us the level and appearance of the oil.

If the oil is black and the level is low, you will need to do an oil change. That is done by putting the engine on an elevated level surface, putting a container of at least half a litre capacity below the drain plug (Item 1 in the diagram), unscrewing the plug, and waiting a long time for the oil to drain (at least 10 minutes). Then put the plug back in and tighten it firmly, so it doesn't leak. Pour about 600 millilitres of SAE30 engine oil in through the filler plug hole, which will fill the sump right up to the top as shown by the line (Item 3) in the diagram. Screw the plastic plug back in and tighten it, using a screwdriver across the two bumps in the plastic.