Radiator Cap - Go to an automotive parts store, get a cap rated for around 7 lbs. or go online to Torrensen Marine, their part number 11499. At $12.10 not too expensive.
Thermostat - Go to the Stant website and download catalog:
http://stant.com/catalogs/filedisplay.c ... t\catalogs Use you existing thermostat for measurements and you can obtain the correct part number. You want a 180 degree unit. Last one I got cost me $7.92 or Torrensen; p/n 24960 at $23.21
End Cap. Torrensen has them for $125.96 (p/n 23169) way too expensive. I have used one you can have for $40. I also have brand new CU/NI heat exchanger made by Sendure for the Westerbeke 50. This is a much better unit than the original since it is copper/nickel vs plain copper and uses a bolt-on metal cap vs the rubber. I want $600 for this and that includes shipping.
I'm currently out of town for the next two weeks, so if you want the cap or the H/E, it won't ship until I return.
Obviously you have an overheating problem, it's been my experience that the parts you are interested in rarely are the source of the problem. Still most mechanics would start with these items. I assume you have checked the raw water impeller and you have flow.
If you'd like to e-mail me your symptoms, I may be able to help. I've got many years of experience working on these engines.
Vern Steele
steelevern@yahoo.com