As said, at £38 you can't go wrong, you deffo got a bargain there. The parts alone are worth way more than that broken. But if you start trying to restore it it will soon turn into 'not a bargain'.
As a tatty runner you could flog it and walk away. If you really want a project think about buying another bike which needs work and using this one as a parts mule. What you don't use you can flog off again to offset the purchase of bike number 2.
If you want more power I wouldn't fart around fitting a different engine to a 400. Kwak already did this and called it the ZXR750
![:smt002](./images/smilies/002.gif)
You can pick a reasonable example up cheap enough without a load of work.
The worrying part about your bike is that it was stored with no oil or coolant and obviously from the state of it not somewhere dry. I would expect that engine to have internal corrosion, the coolant pump to be shagged, bearings to be be seized etc.. That's going to take serious work to get running. It may well run, but as to whether it will stay running...........