"everything" freezing over is a sign that coolant isnt circulating through the convertor(it will have a thin line coming from the rear tank, a thicker line supplying the vaporised lpg to the mixer/carby/manifold, and two heater sized water hoses going to it + maybe some other electrical lines) . The flow through the convertor is necessary to transfer heat to the lpg to change it into a liquid. Without the coolant giving it energy, it takes that energy from the surrounding air, and the body of the convertor, hence it goes icy cold and freezes things up. Sometimes it will freeze coolant withing the convertor if the flow through it isnt high enough.
Firstly, check that your coolant level is ok.
2ndly, get the car running on petrol and up to operating temp and feel the hoses going in and out of the convertor. If coolant is flowing the hoses should both be at least as hot as the top radiator hose, if they are not, then possibly a blockage in the convertor or water pump problem.
If the coolant level was low and you top it up but still the hoses going to the convertor are not warm, then its possible that an air lock has made its way there. One solution is to bleed the convertor. One way to do that is to undo one of the hoses to the convertor at the engine block, do the hose that is positioned higher on the engine, use something to stop the water gushing from the engine where you disconnect it, your finger will do!
Undo the radiator cap.
Put a garden hose into the convertor hose and turn it on gently, run the hose until water exits the top of the radiator.
Stop the garden hose, put the rad cap on, and reconnect the convertor hose to the engine block quickly to ensure that most of the water stays in the hose. Run motor and check for convertor hose temps, recheck rad level.