Oh geez.
I had wanted to do the same thing but found no really acceptable options. Let's take a look at the gun first:
For starters, the foreend of the HK21 is considerably longer (by 5 inches) than the G3. However the casual observer would likely not notice. The second problem with the foreend is it's set up to change the barrel quickly and looks much like an MG42 from the right side, and a slimline G3 foreend from the left usually with a grip that protrudes at an angle. Duplicating that would be near impossible for the average builder. My recommendation: full length rail system and vert grip if you prefer either on the side or pointing down.
The receiver is also a bit of a problem because there is no belt-feed conversion for it in airsoft. You could use a bulky box mag to conceal the magazine well, or if you decide to use G3 magazines you could pass it off as an HK11, essentially an HK21 with a mag well adapter fitted (see pic). Either way a metal body should be a priority.
The buttstock's closest airsoft equivalent would be the PSG1 stock without the cheek rest. That makes battery storage an issue, making the box mag option more attractive if you can squeeze a large battery into it. The SG1 stock could be used I suppose although it's not the proper profile at all.
The outer barrel would need to be heavy if you went with a long version, and the MC51 barrel extensions seen on the market would likely fit the bill nicely. Alternatively you could butt the flash suppressor up against the front sight tower as on the HK21E.
Rear sight would be a problem. Haven't seen anything like it in airsoft and I doubt a real one would fit, nor would you be willing to pay for one. Might be able to replicate something out of resin or aluminum.
In any case, such a project will end up costing you a LOT of money and still not quite look the part. Word has it that VFC/GB-Tech was supposed to be making an HK21 AEG, although they've been saying that for some time now and last report was it was put to the back burner. If you're looking for a good support weapon my recommendation is to check out the CA or Star M249, or other kits that would be easier and cheaper to convert such as the Guarder RPK kit. For the cheapest conversion to quasi-HK21 I'd go with the PSG buttstock, heavy barrel, and RAS, but I don't think you'd be entirely satisfied with the results. Especially lacking a metal body the RAS would probably stress the flimsy TM plastic receiver and eventually break it. Like I said, lots of cash needed for this project. But if you pull it off I'd be most interested to see the results!