Rebrickable has an option to compare set's main models and some of the B-models as well. However there is no inventory for second model for 8285 set available on Rebrickable. The best I could find is a LDraw file available on Eurobricks.
I've imported this file as a custom list on Rebrickable that could be used to compare with sets and MOCs. Imported file gave me some errors (so final result won't be 100% accurate) and I had to tweak this list a bit as there were wrong color or part type used on a file sourced from Eurobricks.
In the end it gave me a number of 83.% shared parts between both versions. Main model needs the rest 17%, which is roughly 320 parts (92 unique) that are not in B-model (excluding spare parts).
I would like to note this set comes in multiple inventories as it known to be initially released with some parts in Metallic Silver, which later were changed to Light Bluish Gray.
List of parts you would need for main model (not 100% accurate, but very close; may have some minor discrepancies due to issues with source file of alternate model):