I've put a notebook fan inside the 1200 case since when I have a 1230/50.
The SIMM goes hot and so the CPU/FPU, so I've put it near the floppy unit blowing air in the accelerator direction. Also some low-profile heatsink glued over the CPU is a nice idea.
But better than anything is swap all electrolytic capacitors around the motherboard. That's a common cause of overheating in desktop Amigas.