I was thinking of replacing the controls with Happ controls. What control panel would you suggest? 
I use 100% Happs controls on my Gauntlet Legends custom panel, and I'm really happy with that.
I'm not playing a lot of Fighting games on it, so Happs stuff is perfect for playing the
classics.
If you're a total Fighting game addict, you should equip your panel with Japanese Sanwa controls

Regarding the panel, the best is to build it yourself of course

You'll be sure that it will fit perfectly your cab'

I've done it on this
post. It's very easy
