Im pretty sure the internal diameter of the 41mm tube will accept 1.1/4" heavy wall underground PVC pipe, we have orange stuff here in OZ, just to help you with the easy install, a good idea when you pull the old spacer out, add about 3/4" to the length for the new spacer. but before you put it in and try to screw the plug back in... use a fine tip marker to put a vertical mark at the top of the fork tube, outside. screw in the plug a little, then carefully unscrew, and at the point where the plug comes free from the tube, put a new mark on the plug where it corresponds to the mark on the tube.
You will now know where the thread starts on both parts.
that way while you are fighting to push down the new spacer and catch the thread, you will be on the money. also it may help to fit a socket and extension bar so you have plenty of shaft to grab, pull or push down, and turn all at the same time.
And I will add the obvious just in case. if doing it on the bike, jack the front up with the wheel just touching the ground, and only do one at a time.
An old friend took both caps off and no jack. Ha, bike fell over, LOL