I will offer a few suggestions, rather than claim the "best turbine" since there are several very good brands available.
You mention it is to replace a 54mm turbine, which hints that the previous engine was one of the Wren ones with a 54mm turbine. If so, Wren offer 3 engines which are in the same case so will drop straight into existing mounts with no need for modification of the airframe and mounts. They are the 75 of 16.5lbs thrust, the Super Sport of 18lbs, and the 100 of 22lbs thrust. I have a couple of gas start Super Sports and a kero start 100. However if you can take the slight drop in thrust to 16.5lbs, the 75 is about to be on special offer at a very good price, see http://www.wrenturbines.co.uk/usa

Harry