Generally, the applicable standards are NFPA 20, "Standard for the Installation of Stationary Pumps for Fire Protection" and NFPA 25, "Standard for the Inspection, Testing, and Maintenance of Water-Based Fire Protection Systems" Your local standards, or those of your insurance company, may be different.
Chapter 14 of NFPA 20 lists several requirements for engine replacement for a diesel fire pump, including:
1. Parts have to maintain the listing of the component when possible, or if not possible, the replacement parts have to meet or exceed the quality of the parts being replaced.
2. When critical path components are replaced (the engine is a critical path component,) they have to be retested by the pump manufacturer, a factory authorized representative, or qualified persons to the AHJ.
Although I don't think that Worthington pump is around anymore, you should ber able to find a qualified company to help you with this. Qualify whoever you find, ask for a resume that shows their experience with this kind of work. Some fire protection companies are qualified (make sure that their technicians are factory trained by one of the pump companies.) Some pump distributors are qualified (make sure that they are factory trained for fire pumps specifically.)
The dealer may have been mistaken, or he may have been referring to another code.
It may be less expensive in the long run to have a fire protection professional look at your situation. Since it has been so long since the original installation, your needs may have changed. You might not need a back-up pump any longer.
By "fire protection professional" I mean:
1. An engineering firm that is experienced and qualified in fire protection.
2. A fire protection company that is experienced and qualified to help you.
3. Your local authority having jurisdiction (building or fire department.) If qualified and experienced with your installation.
4. Your insurance company may help.
Good Luck!