First, put a meter on the battery and see if it's 12V plus. If it is, then pull the terminals off the battery and clean them. Do not just look at them and say they seem ok, pull and clean.
Will it start if jumped? After it's running you should see about 14V at the battery. If not pull it and have it tested.
If no start, then pull and clean connection to starter and the chassis ground connection.
Side note, these will not run without a battery connected. The alternator gets it's field supply from the battery, they are not self exciting. Without the batttery the alternator does not produce any output.