The exhaust ports are terrible for flow, so you want to port those for a start. This really has to be done on a flow bench. If the budget is there, we weld new iron onto the port floors to get a more workable shape, but they can be improved somewhat without that. A one or two mm larger exhaust valve helps as well.
The intakes are not too bad, although they can be improved further. Do not go to larger intake valves.
Combustion chambers can be reshaped to unshroud the intake valves and to avoid trapping fuel the inevitably falls out of atomization getting past the valve, but that would be the lowest priority.
The D cam found in the D-jet engines (both versions) is really quite good and can be had for ~$100 -- I use those in lower-budget engine builds. No need to replace the one in the ES engine unless it's worn out. You can safely run 10:1 compression using premium unleaded with that cam. Of course, you want to fit exhaust seat inserts for use with unleaded.
The other trick, and one ignored by many builders, is to cut down the block deck so you have about .032" piston-to-head clearance at TDC, whatever that comes to with whatever head gasket thickness you're using. If the pistons end up .020" or more out of the block at TDC, that's fine if that's what it takes. This will greatly improve the squish/quench characteristics, which means a much faster burn rate. Faster burn = greater efficiency (both power and economy), less parasitic heating of the head, less ignition advance for optimum power, and less change of pinging or detonation. This is not particularly expensive as long as the block is already apart.