they are the same 'block', but there are subtle differences, such as position of sensors and the oil sensor is blanked off. it shouldnt be too difficult to do it, but look at everything first to see if its there. do the 18se have a crank sensor? stuff like that, you dont wanna be drilling blocks really.
the 18se engine (astra sri) is pretty cheap to be honest and you should be able to find another if you dont fancy it.
thanks for the help I'll have a look first to make sure there are going to be no really difficult sensors to move etc. but they are basicly the same so that's allright. plan on stickin a 2.0 16v or a turbo lump in sometime so dont want to spend to much on the engine.