Anything can be rebuilt. It depends on the skill, enthusiasm and ability of the person doing the work. I am sure, just looking at your broken example that it could be repaired. However. You have to consider the cost to do the work, as opposed to the cost of a unit which can be purchased, serviced and fitted with a new set of gaskets.
There are loads of reports of guys doing different modifications to these motors. I would not be able to vouch for the reliability of the motor after such a rebuild but generally, I am lead to believe that they hold. However, mine, with it's standard carb and pipe setup, is more than able.. I wouldn't even consider modifying it. But then, I am just over a year from fifty and have been riding bikes all my life. The bike is a 2001 and has had a lot of mileage put onto it. I am not sure how much. Stripping it to try and find a noise, revealed a really strongly constructed motor and gearbox. The wear was so minimum that I would say it has done 10% of it's lifespan already. Though I know it has high mileage, I am confident it will last me till I die, even if it was the only bike I owned.
I am not sure whether there is a ny difference between the head of an E and an S.. I am inclined to believe not. However. If you contact Suzuki and ask them if the part numbers differ, you should be able to start to work it out. As far as I know, the exhaust and the carb is the only difference in spec between the two bikes. I am lead to believe the cams are the same as well...