Here's the TPA3106D1 datasheet. The application circuit on page 14 is allegedly mostly (or even exactly) the same as the 44 Magnum.
Here's a full project write-up for a pedal-sized amp based on a similar chip. (It's in Italian, but you'll get plenty of information from the pictures. If you want more, there's always google translate.) Both of these are SMD, though, and not really doable for the average DIYer.
There are plenty of other chipamps that are a little easier to work with. (
Here are a few to explore. ) Beware, though. Just because the 44 Magnum looks like a pedal doesn't mean that pedal-building skills are sufficient to make a power amp, even a solid-state one. Power amps and pedals are not really equivalent. Layouts for large-ish power amps have to be carefully optimized to avoid oscillation, and most of the engineering goes into the power supply, not the amp circuit itself.
If you want something reasonably similar, there are some decent projects on DIYSB and FSB. The
Tiny Giant comes to mind.