Out of the water for most jobs I'm afraid. Change oil anualy if you do plenty of hours running, change gaiters every couple of years, lube steering linkages the best you can. It all depends of how well maintained they have been in the past as to how easy it is now. I'm assuming enfield 130.
Use EP90 oil, an enfield 130 should take a gallon to fill from empty, let it drain for a good while if cold. Make sure you remove the drain plug and not the pivot screw for the gear linkage as you'll probably need a strip down to get the bloody thing lined up again. Oil drains much quicker if you remove the filler cap (on top), filler is easier to access if you unscrew the steering ball from the steering arm but don't cross thread when you put it back (3/8unf I seem to remember), when you fill wait till it comes out of the level hole and let it settle a good while to get air out of the lower case and re-check the level, a little bit of water in the bottom isn't unusual, a lot is a problem.