Hi Malc,
Don't throw anything in the bin! I'm assuming you have an MJ Engineering design 3" Fowler? Certainly join a local model engineering group or sign up to Traction Talk, plenty of specific advice from boiler inspectors there.
If the engine was built in 1991 and has no evidence of a boiler cert then I would be tempted to borrow a hydraulic test kit and have a go yourself at pressurising the boiler yourself, just to see what happens. If it stays dry then there's no reason no to steam test it. Remember, a hydraulic test is at twice working pressure which must hold for a minimum of 10 minutes without a significant drop in pressure or and buckling or structural failing of the boiler.
The boiler inspector will require a boiler serial number, usually stamped at the bottom of the back head or on a plate stuck on top of the fire box on the outer wrapper. If there's no serial/ID number then I believe you're able to put your own stamp on it but I can't be sure? One of the boiler geniuses will be able to tell you on here.
With regards to a steam test, the safety valves also need to lift or feather just before the pressure reaches working pressure. The pressure must hold at not more than +10% of the working pressure otherwise the safeties will have to be adjusted (which is a nice job to do whilst in steam with a fowler!).
Finally, modern regulations state you need at least 2 methods of getting water into the boiler. Be it by mechanical pump and injector, twin injectors, mech pump and hand pump or injector and hand pump or a combination of all 3! So make sure you've got them and make sure they work!
Hope some of this helps!?
Regards,
Dan