990 Head Gasket gone?

I fired up the track bike yesterday, for the first time in a couple of weeks and it didn't run cleanly initially, with an erratic misfire for a few seconds before clearing and settling down. The first couple of minutes of running were accompanied by clouds of water vapour from the exhausts, and I noticed that the water level in the coolant expansion tank had dropped. While the engine was running (temp gauge half way up the scale) I tried removing the radiator cap and coolant sprayed everywhere as if the system was fairly well pressurised... After the initial misfire the idle and throttle response were as normal, no more water vapour from the exhaust.
I shall have to investigate further, looking at the expansion tank. After it cooled down I saw that the radiator level had reduced a little but that could be due to the losses when I tried removing the cap. I may have to strip it down a bit to remove the plugs and see if either is wet, and maybe poke a bit of cloth on a length of wire down the plug holes to see if the piston crowns are wet if the plugs are dry. I don't even know where I can turn the crank over with a socket, it's all a bit new after a lifetime on Japanese bikes.

Question: if I do need to replace a head gasket, can it be done with the engine in situ?

Unlikely to be the head gasket, stick the bike on it's side and pull the oil filter. If the fins are wavy, then most probably the water pump seal has gone. It's a cheap and easy fix.

Thanks for the reply.
Water pump seal failure wouldn't explain the misfire and clouds of water vapour from the exhausts unfortunately (much more and for longer than you'd get on a cold morning, and this was on a mild afternoon). I wish it were as simple (and cheap!) as seal failure but I fear that it's worse...

Wow - sorry to hear any of that?

Bike always produced a bit of white smoke when first started after a layup but it cleared quickly and then never reapeared at all.

Water pump seal was done by Projex shortly before I bought the bike, it never used a drop of coolant while I had it. I did top off the expansion tank a bit as I had a bit left over in the bottle since it's last ride but nothing that would have made any difference.

Does it just need a bit of a run? hasn't turned a wheel since October last year....

Call me anytime if I can assist.