The diodes are super cheap so it is worth the shot to repair it.
Almost any rectifier diode will work, the most common is a 1N400x, the x can be any number, it don't matter. These used to be available at Radio Shacks, with the recent reorganization of RS I do not know what they still carry, my local one is one that closed.
The diodes are available from Digi-Key for $1.75 for 10, you need 8.
http://www.digikey.com/product-detail/en/1N4006G/1N4006GOS-ND/1485478
The shipping will be more than the parts.
Once you replace all of the diodes, plug the TCI into the bike and make sure all pipes are getting hot at the same rate then go thru the effort to seal the TCI back up and install properly in the bike.