I do not know specifics on the Flyer's Composite monitor. However, I know my way around video assist and trouble shooting monitors and signal. Francisco is correct the Decimator MD-HX is not what you need. The decimator 2 would be my recommendation as well, it is also a favourite of many Steadicam Operators I know. AJA could be another option for you. Personally, I am found of them as you can not only down-convert but split HD signal as well.
I am not sure how much trouble shooting you have done thus far, so please forgive me if I touch on things you have already tried.
Elimination is the key, start simple and figure out what works then test the system to find the fault. It could be the down-converter, the Flyer monitor, a cable, the wiring in the rig, etc.
I would start by bypassing the rig and wire the monitor directly from the down-converter and see if that helps. If you have access to another monitor which accepts standard definition signal (tv logic, small HD, Marshall, Panasonic, etc) you can use it to make sure the SD signal is working from the down-converter. If you try multiple monitors composite inputs and they do not work, it is the down-converter or possibly the cable coming from the camera to the unit. If you are able to get an image on another monitor and not yours, it could be an issue with your flyer monitor.
Once you know the down-convertor works, and if you can get a signal on your monitor with a direct line from the down convertor. Try wiring through the sled, if that does not work there could be a problem with the internal wire or the connector on your top stage.
I have recently had to down-convert a Sony A7 R2, not having access to a down-converter with an HDMI input, I used a Small HD 502 monitor to convert the HDMI to SDI then ran a BNC to the down-converter.
Hopefully, I was somewhat helpful.
All the best and good luck!