I am looking for an aftermarket set of headers with collector.
I need large diameter pipes and I need the pipes to enter the collector with 1 and 4 being opposite each other, not next to each other like the stock collector.

Any help will be much appreciated.
Thank you.

That sounds like something you'll need to get made specially.

As Rick says. To my knowledge there is nothing available off the shelf even at places like BSK Speedworks.

Maybe this will help

Caferacer parts – Tagged "exhaust"– KpartsHolland

Brrapp wrote:I am looking for an aftermarket set of headers with collector.
I need large diameter pipes

Why? Did you increase before the valves diameter, the ports, the bore, the opening camshaft duration?
If not, larger pipes will just decrease performance. Ask Bernouili.

Brrapp wrote:I am looking for an aftermarket set of headers with collector.
I need large diameter pipes and I need the pipes to enter the collector with 1 and 4 being opposite each other, not next to each other like the stock collector.

Any help will be much appreciated.
Thank you.

There was a member here, years ago, who cut and welded all of the bits together himself to create a wonderful snake pit of header pipes. His name is Scheffers, and the link to his complete post is HERE , but you'll see that the pictures are no longer a part of his original K100 Forum thread. Perhaps a quick question of the Google Gods will reveal a close-up image of them for ideas.

HERE is a link to an article about the bike from motorcyclespecs.co.za...

Next, and I am considering JBT's pertinent questions to you here, too, we all have our reasons for wanting this or that, but assuming power is what you are trying to make more of, consider that even UK-based BSK Speedworks' full-on racing bikes use the stock headers with their aftermarket collector, and a custom cannister. 

If it's simply a fitment issue you're attempting to solve, custom will be the way.