Oil pressure you will need a new sender. Given it's driving a light, it will operate on an on/off basis, with no in between. (From memory they illuminate at 7psi?) I'd get an adapter so you can have both a gauge and the light functional. Best of both worlds.
Water: Maybe - I'll defer to some of the gurus here on whether it's a definite. The key thing is the sender operates on the same range as the gauge you add. I don't know if this is standardized or not. (ie, 2 Ohms of resistance equals xx degrees etc)
Personally, I'd just buy a new gauge and sender then you know it's spot on. I prefer mechanical water temp gauges (capillary line) as they operate without ignition, although each to their own. You may need an adapter to go into the cylinder head depending on the thread on the new gauge/sender.
See
http://www.speco.com.au/gauges_ad.html for adapters. You will need to work out what threads you need.