Quick check of the Challenger website - they don't sell an ELSA. So what it might be is a "fat ultralight" that went through the process of getting an N-number and ELSA airworthiness certificate. Problem is, ALL of those AW certs have expired. So if it is an ELSA with expired AW cert, then it's parts, not an aircraft legal for flight. Sorry. Check the airworthiness certificate of that airplane. If it has an expiration date that is in the past, it's not legal and can't be made legal. Parts.

Another possibility, though, is it has an E-AB airworthiness certificate. In that case, for the tail mod, check the log book that there was some documented flight testing. Another thing to check are the Operating Limitations. This is a document that the inspector issues with the AW cert when it's inspected. This will have a paragraph describing the process for documenting test of major modifications.

You will be able to get liability insurance through USUA, if gross weight is 1150 lbs or less (I couldn't find gross weight of a C II on their website, but with the engine choices listed, it's most likely in the 1000-1100 lbs range). This is liability only, doesn't cover a passenger, and hull (vehicle value) is also not covered. "Real airplane" insurance companies probably wouldn't touch it. But if you need liability to put it at an airport or in a hangar, going through USUA is not difficult or all that expensive (~$500/year or less, depending on weight).