You need to find the distributor. The parent company will be worthless. I am looking to replace a Trumpeter part and found an MMD-squadron sticker on the side. That is the distributor for many Trumpeter kits and also might do Hobbyboss.They are still slow to respond but they usually do.
Like Jeff just said, there are always options. I you have the part(s) number(s) you are missing, this will help on may levels. As for the distributor, MMD is one, Steven International is probably the next largest. Other larger companies like Squadron and Megahobby have a foothold as well if all else fails.
Also, if you can, snap a pic of the part from the instructions...some folks here have built this and may be able to help as well as maybe a way for you to scratch build the part.