You'll destroy a Dremel.
Buy some extended carbide burrs like these:
You need a grinder, even small compressors will support a grinder (They say not too however), use it in small spurts and lubricate the burr with WD-40 constantly to keep it cool. Go slow and set A LOT of time aside for this, roughly an hour for each chamber.
As for porting itself, start first my just looking for and correcting casting flash, then move onto the unshrouding the valves, then finally look at the ports. Air likes to go in a straight line, if a corner or bend is required in a port, try to smooth out the transition as much as possible.
If you want to open up the intake or exhaust ports, you will need to put the head back on the block, apply some machinist's bluing, seat the manifolds and tighten them down. Then remove them again and notice where the bluing remains and where it transfers. This will tell you if the ports line up and how much you need to open them up.