There were mentions of porting the back of the unit a while back on one of the US forums - having actually tried it, its feasible, but yes, a complete pain in the ass - And not an exact science, I ended with a unit doing 270fps on propane, but then you've got cycling issues beyond belief.
PDI 6.08mm // Orga Magnus 6.13 or 6.23mm. AEG type barrel. Length wise 233mm. Regardless of which route you go, you are looking at £75-100 of inner barrel in a £100 rifle. Having spent a lot of time, money and effort testing various widebores in various platforms, your best bet is likely to be an Orga MagnusHD 233mm length, 6.13mm diameter - which are roughly £75-90 dependent on the source, but that will bring you to about 330fps.
IMHO, sell the gun on, the extra cost to polish a turd just isn't worth it.