I have just got my ZXR400H2, have riden it once and now, before I've even had time to introduce myself here or even take a decent pic of my bike for your amusement, I've got a serious problem. Another £870 Ebay wonder

Towards the end of the ride I noticed I was doing a passable impression of the Red Arrows with the white smoke on. Bike seemed to run ok but was leaving a touching vapour trail behind me (hadn't even had a curry

This morning, I started up and left the bike to warm up as I wanted to check the temperature gauge / radiator fan. When the needle of the gauge had got to probably 4/5 towards the red a worrying burble / rattle seemed to be coming from the top of the engine, albeit the engine was still seemingly running fine. Then I noticed a huge puddle on the R.H.S. of the bike accompanied by a boiling type sound again from the top of the engine. The fan had not cut in at this point and it had taken quite some time for the engine to warm up to this level.
I switched off, water continued to pour from the expansion bottle overflow pipe and the boiling noise continued. So, have I blown the head gasket or is there another explanation? The bike is a 1989 H2 imported from Japan (I believe) with 37941km recorded but it does have a KPH/MPH converter (in fetching pink) so I'm not sure what the true km/mileage really is.
If the head gasket has gone, where can I source one from and is it a fairly straight forward job?
Can I do it with the engine in the frame?
Thanks for all help, suggestions etc.
Mick.