I know and I would never inject CO in my tank. I'd love to convert CO
cylinder to CO2 system

I dont know why do you want to do that? CO is very dangerous even at low
levels. If you are looking for a cheap system have a look at my experience
http://www.dlink.org/aqua/CO2.html

Regards
Dominic

PS: I am planning to sell my CO2 cylinder, Regulator and other items in a
few days.