Hartley was bought by guys in the UK, but the Radical was previously the Powertec Engineering V8. They changed the name because they install it in the Radical chassis they build. Hartley was bought by his UK distributor who tunes motorcycles and was building turbo versions.
The RP V8 differs in several ways from the Hartley. It has a cast crankcase whereas the Hartley was machined from billet. It uses balance shafts and has a very different exhaust note suggesting a different firing interval between cylinders. Attached are two brochures, one when the engines were called Powertec and one as Radical Performance Engines.
The Powertec motor was developed a couple of years before the Hartley. I don't know how many Hartleys have been produced, but Radical has sold 116 of their engines. I followed the development of all three bike derived engines: the Powertec, the Hartley V8 and the RST which was originally developed by Mountune Racing.
The Hartley and the RPE were both based on the Hayabusa, while the RST was based on a 1000cc Yamaha FZR. The RST has since been stretched to 2.4L and supercharged, whereas RPE has developed versions of the Radical from the original 2.7L up to 3.4L. A version of the RPE has been chosen as the spec engine for the Argentine touring car championship.
Powertec-Macroblock-v8 - race-winning engines