I just went through this whole exercise
*New Ork torsos work just fine with old Bike legs. But Ork Biker legs from the new Kit are pretty reasonable, Loota Legs also ride bikes well, Some of the plastic NOBz also are 'non-walking' so they ride bikes well.
*Consider getting a box of the NEW bikes and mix and match parts. You get a few extras in the Bike box.
*'Cowcatchers' is something new for the modern bike. It doesn't take much to make one out of bitz. Having front bumpers and scythes from extra chppas can break up the look as well as modernize.
*Plasticard I-beam + Boyz sprue ammo belt = Replacement for goofy oversized bullet ammo feed.
*Replace the goofy Evil Sunz face with literally anything.
*When in Doubt, Get new guns. Loot from extra Big shootas from other kits, or extend regular shootas.
*Shoota 'cup' hands + Choppa Arms = Holding the handle for new modern boyz.
I just rehab-ed like 20 of the old bikes, 10 space marine bikes and 20 of the new bikes that I got off ebay for less than 5$ a bike
Also, if you need a warboss, but don't want the
FW warboss, this guy rides bikes PERFECTLY.
http://www.ragingheroes.com/products/orc-warmaster-science-fiction
I also Highly recommend the Forgeworld NOB kit. Not only do you get 3 NEW bikes which come with a full set of 'bitz' but you get 3 Extra tires, 3 Extra Ammo Feeds, A Really great 'TRIKE' set up which is really good for a warboss and 3 NOBz who are perfect for bikes (who come with 4 arms for handle holding since the double holder on guy will probably have a
PK). If you drop the money for the NOB bike kit, and have bikes who need some updating, it is a great option
IMHO.