The reason voltages over 77 or so are considered "getting dangerous" (and the rules change at 50 volts) is that skin resistance is usually so high (even when sweating heavily) that such low voltages won't normally push enough current to stimulate muscles and nerves even if you grab a hot wire, while voltages in the 75+ level usually will push at least enough to make your muscles twitch, and potentially could do that to your heart if you grab on hard with your left hand.

But you have the same problem with AC and both electricians and do-it-yourself handymen work with it all the time.

The rules are virtually identical for AC and DC at house-wiring voltages:

 - Only handle unpowered wires.
 - (Be VERY careful and keep your left hand in your pocket if you MUST work in a box where anything is still hot.)
 - Don't grab hard onto anything that MIGHT be hot.  (Even if it's supposed to be dead touch it before grabbing it.  That way the "twitch" won't make you grab on harder if it turned out to be live.)
 - Don't get into a narrow space with exposed hot wiring.  (Risk is being thrown into continued contact by uncontrolled muscle action if you bump something hot.)

Given the pix it looks to me like you should have no more risk of electrocution working with this thing than with working with an AC system of the same vintage.  Main risk looks like the exposed knife switch and the narrow space behind the panel with the hot stuff back there.  So don't go behind the panel while the genny is spinning and keep the area in front of the panel clear of tripping hazards until you can get a guard over the knife switch.

= = = = =

I'd proceed by:
 - Tracing the wiring (and examining it for anything that looks like bad insulation)
 - Getting the genny to turn freely.
 - Maybe "stoning" the commutator.
 - Fixing up any wiring that looks like it needs replacement.  (Note about old wiring:  Flexing it may make the insulation flake off, so don't disturb it unless you're replacing it, then replace everything you have to bend (except the enameled wire in the motor/generator terminal box).  That means pulling new stuff through conduits if necessary.
 - Spinning the genny with a motor while getting the genny/control panel working under no load.
 - Going up to full operating voltage and lighting a small load (like a nightlight bulb.)
 - Hooking up the chain.  (Don't want to put multiple horsepower into it until you're sure it's not shorted.)
 - Loading up the system inside the shack (with new, temporary, loads, like a few incandescent lights).
 - Inspecting and fixing up the wiring to/in the cabins before powering THAT up.

Of course there are lots of points in there where you can determine if there's a problem that means you should stop now.  (For instance:  No point in working on the cabin wiring if the genny isn't going to work.  No point in hooking up the genny to the wheel if the genny is shorted.)