It depends on whether you have a URT design or not. If it's URT a conventional tensioner will work, as the hub to bottom bracket distance is unchanging.
Otherwise I'd use a cheap short cage road mech, like a Shimano Sora.
Anything with 2 wheels making the chain serpentine or with sprung tension that has the capacity for the change in chain length .
Maybe an old Goldtec Manacle ?
Id be tempted to try an mrp device up front with a sprung tensioner on the mech hanger with some cut down sprockets sandwiched between the one sprocket at the rear .
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