This chart might be some help: (if you get determined to change the forks)
http://scandalon.com/2009/06/motorcy...iameter-chart/
I'm *suspecting* you/we are dealing with a 27mm tube (measure what you have to be SURE).
I have put 35mm tubes into a 33mm set of trees successfully, I don't *think* you could stretch a 27mm tree to 30mm, though as you can see, there are plenty of 27mm forks to choose from, just watch the length.