Try it out of the case if it starts up then. With ipmi working but nothing turning on when you hit the power button, it might be short circuit protection. There may be spacers that touch the board if the old X8 had different spacing for the screw holes.
$30 for a 3D printed 2.5" to 3.5" adapter, lol. That’s like 30 cents of parts and labour right there. You can even buy those made of steel for a few bucks because it’s so easy to make…