Coolant.

i had a look at that evans site and unless its a new engine or a very clean engine i wouldnt even waste my money buying it, how about you drain the coolant out flush it well, then put plain tap water in and run it for a few hours or a couple days and drain it then maybe try that again, oh and while you are there try to remove the engine coolant drain plug thats on the left side a foot down from the manifold, and flush it out good as most shiit will sit in the bottom of the water jackets, thats what i did and i have clean coolant now, which reminds me i need to change it again as its been a few years, i usually do mine every 12 months to ensure the system stays clean and corrosion free.

Tony.

weld some big fins on the block and make it air cooled

Just bunged some ferinox in it. I'll give it another flush next week.