I've finally managed to remove the cotter pins and bottom bracket. The cotter pins were a real pain.

Is it possible to fit a cotterless bottom bracket / axle or are there any bmx 3 piece bracket / axle cranksets that will fit a grifter?

Anybody know of any alternatives?

Just wondering for any future reference  

it can be done, i've seen a pic of a grifter on here somewhere with one of the mk3 chopper crank on, not sure if it's straightforward no doubt someone will tell you more ?