Chapterhouse Studio have released their Farseer on Jetbike conversion kit for $15.50 US.
From their website:
Games Workshop Jetbike kit not included – necessary to assemble as seen.
This is a 12 piece resin conversion kit that when added to a Games Workshop Eldar jetbike will form the model shown. It is perfect for converting a regular jetbike kit into a Farseer on Jetbike model.
Each Farseer Jetbike Rider kit comes with 2 weapon choices, 1 left arm, 2 upper torso choice (male or female), 1 lower torso,1 Farseer head, 1 control panel, 2 shield generators and 2 bike accessories.