There are a million reservoirs you could use on that bike. Ideally, you will need a 1/2" or 13mm master cylinder and make sure it has a brake light switch. There will be thousands of options if you go the salvage route, but if you want new you only have hundreds
IMO you could get away with a 11mm or 12mm m/c too, but why confuse things more, right?
Option 1: buy a new OEM m/c for a Hawk from Honda, of course that will cost you $220
Another option would be to get the complete assembly (m/c set, lever, harness, from a TRX450R for like $60. The lever is even adjustable. (Note: You may need a new brake line for this)
Yet another would be to get a Brembo master from yoyodyne. They have a 13mm integrated res m/c for about $80. Then you buy the switch for about $12 and you are all set. Bonus of Brembo is that there a lot of different levers avalable from the solid 900SS style one for $11, to the over $100 adjustable folding deals. The final bonus for the Brembo is that you can use the same brake line, whereas most new Honda m/c will have the banjo oriented toward the front instead of the side and require a line with a different fitting.
Try this:
http://www.yoyodyneti.com/detail.aspx?ID=225
+
http://www.yoyodyneti.com/detail.aspx?ID=321
If you want to get fancy you can special order a 13mm remote master, but those can be hard to find. I would recm'd you don't get a remote res m/c from a junkyard because there is about a 99.9% chance that it will be a 16mm m/c which will not work real well with the stock Hawk caliper. It probably feel a little wooden and will definitely feel less powerful due to the reduced leverage.
Good luck.