Welcome to the Red & Black road. It's a journey that will empty your wallet on occasion, but you end up with a nice collection of HK firearms.
Again, welcome to the HK ENABLERS forum.
Where did this HK91 come from, and by that I mean from an individual or a company?
Not sure what you have, but there are a lot more knowledgeable people on this forum that will speak up. Give them until the end of the day and then see what they say. Their knowledge will surprise you.
I own an HK91 IB and it's all correct as it were. But I did this careful search six/eight years ago. It wasn't any easier then and I just rolled the dice and got lucky. You might be surprised, but you can find MOST of the needed parts with correct date codes, but it will cost you.
Besides, you have not told us why you purchased this. Safe Queen? Every day range rifle? Lots of things go into this equation.