you really need two flat wrenches for the headstock.
it is a 1-1/4". your options would be to dedicate a Crescent Wrench
or two open-ended wrenches. you will have to grind down the faces
a little to fit between the machine and the faceplate, chuck, or whatever.
some chuck nuts are bigger and a Crescent Wrench will work fine.
check your local pawn shops to see what you can modify with a grinder
to make it work. with a 4" angle grinder and a metal cut-off blade,
you can cut up an old lawn mower blade to make a really nice wrench.
good luck in all your projects !!
the reason I know so much about this is that on my last home move,
I lost the wrench. it drove me nuts every time I had to remove something
from the headstock. I don't use my lathe much anymore. but it is nice
to have all the appropriate tools when you need them.
a quick E-Bay search turned up this one for $10.00 with free shipping.