The RB Speed .21 engines are essentially a reissue of the RB C6 CC engine which was a very good 7 port engine, it's got plenty of top end and a reasonable bottom end, for the price it's a very good engine.
RB have discontinued the RB Speed though, so you will not be able to get a new piston/liner for this engine any more, rod and bearings are still readily available though as they're used in other engines.