Federal rules required that your car meet FMVSS (U.S. safety standards), which include needing bumpers, when it was mfgr'd., sold, or imported in the U.S., but those requirements have expired as of the car's 25th birthday. [49CFR30112], IIRC.
However, you must also meet the state's equipment requirements, as specified in state law (statutes?). This may dictate a certain bumper, or it may not. I'd recommend you read over the pertinent section of the law, comply as needed, and appeal the cop's decision if appropriate. The cop can only enforce existing laws, like it or not.
In Wisconsin, such an appeal can be done (with or) without an attorney through our Dept. of Administration.
Good luck with this.
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