wow that has had a hard life
cosmetically it could be fibre glassed/plastic welded, i've had good results plastic welding using something as simple as a soldering iron and off cuts of abs plastic, there's plenty of tutorials on youtube that could get you started off, if you haven't got access to spray equipment such as a HVLP (high volume low pressure) you can still achieve good results using rattle can (aerosols) the frame and forks can be aerosol ed too, or worst case scenario brush painted to smarten the old girl up a bit.
engine wise if your looking to replace the 400 for a 600 to for want of a better word "gain more power" but your on a tight budget how about servicing the engine and repainting using PJ1 or a similar engine coating to smarten it up then look at altering the gearing to give either more acceleration or a higher top end?
braided steel brake lines will improve the brakes rather than sourcing blue spot calipers.
bottom line would be in my mind, make it presentable and safe with the least amount of money possible, elbow grease is free so your hard work and dedication will not only get you there but it will be something to be proud of when finished if its as much of your own work as possible.
failing that its a money pit waiting to happen i reckon if you want the 600 engine etc
whichever way you choose, good luck with it mate, you got a good few hours of tinkering on your hands there.
