The best replacement for histidine is imidazole.

The best alternatives to histidine are rated as following:
★★★★★  imidazole
★★★★  histamine

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Speaking about histidine, one probably shouldn't forget about beta alanine.

Other search terms related to histidine and their ratings are:
★★★★★ beta alanine
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histidine
imidazole
histamine