Can I generate a clock pulse by using just one SPST switch?
TTL inputs default high, and 7490's clock on low-going edges, so all you have to do to clock one is to connect one side of the toggle switch to the 7490's clock pin and the other end to ground.  Then, every time you toggle the switch from OPEN to CLOSED, you'll clock the 7490.

The count will be erratic, however, because the moving contact in the switch will bounce on the fixed contact and clock the 7490 many times until it finally comes to rest.  To clear up that problem you'll need to debounce the switch, which is a whole 'nother question.

Also, good practice dictates that if you're driving it with a switch you should connect about 4.7k from the clock pin to Vcc for bipolar TTL, which is what a 7490 is.