Hey Blumbly,
Don't worry - I wouldn't run it with the spark plugs in place. Just to test and then I would fit new (Or rebuilt) decompression valves. I don't believe in make do repairs - If it's worth doing it's worth doing right.
I know people do very stupid things in the view of a quick result but I would love the idea of my machines being handed into a museum in the years to come not a scrap metal pile.
Thanks for your help. I'll report back when I have more news.
Pete.