Not sure if this is what you want but...
it's easy to convert shootas to big shootas. You simply need three sizes of plastic tubing, 3.2, 4.8 and 6.4
mm from memory. This is how the fella on the right was done - I simply sawed off the barrel of his shoota, and replaced it with the tubing.
The boy on the right is converted from a shoota to a big shoota. The boy on the left is converted from a slugga to a shoota.
The fella on the left is converted from a slugga to a shoota, using the shoota's sawn-off barrel. You need to cut and greenstuff the wrist and shoulder, as the slugga arms otherwise won't curl across the body as necessary. I haven't greenstuff'd him yet. Yes, I know their left arms are slightly out of position - they dropped while the plastic glue was drying, grr...
I bought around 5 tubes from Howard Scenic supplies for £3.15, that would be enough to convert literally hundreds of shoota or big shoota boys. Hopefully they will work for bike bits too.
Apologies in advance if these are boring and not what you're looking for...