This is the old Burnham Holiday boiler I will be working on.
Before we start, you should really consider having a real professional do this job for you.
I understand the cost problem, so I wont go into that.
Here are some of the reasons you should not try to do this yourself.
1- Gas is explosive.
2- You may have a Carbon Monoxide problem, that requires a whole boiler replacement.
3- You may not use the correct gas valve.
4- Gas valves need to be setup with a water column gauge, that you don't have.
5- The boiler may need many upgrades, because it is so old, and not having a professional look at the boiler, will leave you in the bark about real safety issues, that are very likely with an old boiler like this.
There are other reasons, so please think about it.