can I use my 2009 iMac as a monitor for a mac mini? the mother board is bad on my 2009.

According to this support article https://support.apple.com/en-us/HT204592 you need the following:

I forgot to mention, that in order to do that you need to start the iMac in target display mode, I don't know if you will be able to do that if the motherboard is fried 😟. Please read the support article to make sure