You could take your Japanese Dreamcast apart and replace the broken parts with the working ones from your PAL Dreamcast
Edit - ...well, this method obviously depends on what's broken. The motherboard should be the Japanese one, everything else (including the AC adaptor) can be taken from the European PAL one.
Edit2 - SWAT's fantastic Region Changer v1.5 should also be worth a look, though personally I would recommend to try to repair your Japanese Dreamcast at first.
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