I found the schematic for this at DIYstompboxes and thought I'd give it a go. I think "Metal" is a bit of an exaggeration, but it sounds pretty good considering the low parts count.
The circuit appears to be a simple common emitter boost pushing a bazz fuss. The tone control is a simple high-cut filter, but it does the job.
Feel free to experiment with the diode type. The original specifies a LED.
Also, using a higher value gain pot gives more gain in the first stage, which means more dirt. A 500K pot provides a decent boost in overall clipping, and allows for easy pinch harmonics. 1M might be even better, but my gut tells me that the difference between that and 1M would be on the small side. But feel free to give it a go.