You'd be surprised how easily a custom motorsports shop can whip one up for you.
If you haven't toured your local shops, I'd give it a shot. Or if you find a good deal on a used header, like the ones you've been looking at for other engines, even lesser equipped shops can modify and weld on a 1.8 head flange for much less cost.
I'm really excited to see the first 1.8 boost conversion.
There was a low boost 1.8 90's Volvo wagon running 13.6 all night last time I was at the track and I was sprung on it.
I'm not suggesting you do the conversion for all out power initially. But once the details get ironed out, you will have inexpensive options to power up very effectively.