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SurveyJS
Open-Source JSON Form Builder to Create Dynamic Forms Right in Your App. With SurveyJS form UI libraries, you can build and style forms in a fully-integrated drag & drop form builder, render them in your JS app, and store form submission data in any backend, inc. PHP, ASP.NET Core, and Node.js.
This is the one (rattletrap) that can convert it back to replay as well (harder to read): https://github.com/tfausak/rattletrap/releases/tag/11.2.4
Then you can try to find the decal id either by searching "decal" from the rattletrap json. Or from the other json by "SkinProductId" (player name will be a bit above it) or just with player name couple times to see if there's "TAGame.PRI_TA:ClientLoadouts" after it. Finding stuff will be easier with https://jsoneditoronline.org/ tree view I've found.
This one will be easier to read (RocketLeagueReplayParser), and you can find the id's of desired decals and which player they belong to etc: https://github.com/jjbott/RocketLeagueReplayParser/releases/tag/v2.7.2