The difference between a sole source and a generic spec is mostly in the use of brand names and model numbers.
You can specify the engine and transmission brand since they are available to any apparatus builder. Same goes for most controls, valves, etc. This would be done to "ensure compatibility with current equipment, reduce training variations, and reduce spares required, maintenance training," Thus we specify Allison transmissions on custom chassis. You can specify cabinet sizes or what has to fit inside. If you are furnishing equipment, such as SCBA and spare bottles and want a particular rack installed you can specify that. You can even specify wheels and tires Pumps are much more generic in design so all you usually specify is capacity, discharge/intake locations and sizes.
Look at your current spec and see if deleting brand names for features and or replacing them with functional specifications will work. Our spec always includes a "hill climb" performance requirement because we have a notorious hill in town and the apparatus must climb it to spec to be accepted.
Your solicitor can advise as to proper language for your jurisdiction. Or you can hire a reviewer. The less detailed the spec the easier it is for a builder to respond, the harder it is to evaluate, and you mioght be ambiguous on something you really want your way.