I own a 1993 SR5 pickup V6 4WD, and it had a cam seize up. Is it worth while to replace the cam, or should I just replace the whole engine?

Your cam probably seized from lack of oil. So if you did replace it, you'd probably need to replace your oil pump, if the engine has low miles on it you should be able to just use the sprocket and chain off the old cam, make sure you have a manual from autozone or a similar place to check the valve clearance. It would be easier to replace the cam than the engine and cheaper.

The engine has 230,000 miles on it, and my mechanic told me that the cause of the seized head was that oil was not circulating well, but the oil pump is in perfect running condition.

That means its got a clogged oil passage somewhere. Those are usually hard to find, you'd most likely need to disassemble the entire engine and clean it. If your not experienced with removing and reassembling an engine I wouldn't rec amend it. @ everdrive.com there is a 3.0L for you truck with free shipping and a 3 year unlimited mile warranty for $2,300.