I understnd that, mate but with very high pressure gas, you just can't feck about with the wrong kit. It will end in tears. At best you'll ruin your gun. at worst you'll ruin yourself.
Jokeing aside, as Archie's said if you can't stretch to the proper gear, sell it and buy a decent springer
What Im saying if my welding bottle if safe to use it would last me alot longer as it is a much bigger bottle than your diving bottle