For the front, a tube with a range somewhere in the neighborhood of 1.1-1.5 should work. For the rear, more like 1.25-1.75. Neither has to be exactly the same as the listed tire size.
Future reference, your 20" wheel is a 406, as opposed to 451. 451 tire sizes are listed as fractions, as opposed to decimals. (Almost always.)
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Bacchetta Giro A20, RANS V-Rex, RANS Screamer