Grab the 3.50 Downgrader from Nobbz Homepage (
http://www.noobz.eu).
You will need an unpatched copy of the game Lumines, but even so, it will be cheaper than the motherboard replacement + fitting costs.
As Fanjita said, using the downgrader will be easier and cheaper.
As this current time you would be stuck just if your PSP were 3.51.