Clark made use of the Continental L-Head engines, Fourcylinders models N56, NI62, Y69, Y91,Y112, F124, F135, F140, F162 and F163.
Six cylinder models F186, F209,F226, F227, F244, F245, M271, M290, M330, M363, B371, B427.
Normally an engine name plate is attached to the cylinder block, that would help identify model and serial number (wich could be helpful with ordering parts).
I have no expierence with ordering online parts, googled for continental flathead engine parts and get a lot of hits like continentalengineparts dot net. Should be a oiece of cake :)