Some easy and safe things you could do (if it's not already done on your Force):

Cut away some of the skirt on the piston to allow for better flow from the crank case to the transfer and boost ports.
Knife edge the lower part of the liner also to allow better unrestricted flow to those ports.
Check how the ports in the liner mates up with the ducts in the case. If you can see a mismatch you could grind the case to get a good fit and better flow.
Take out the sharp edge on the lower part of the transfer and boost port so there is a smooth transition.
Some people recommend to polish the outside of the liner, I'm not too sure it actually helps but I've done it and it didn't hurt anything.
Make sure yoy clean everything very carefully afterwards.

Those things are safe and easy to do and doesn't change any timings in the engine, it just improves the flow. I've done it on a few and I got a bit better fuel consumtion and for some weird reason it ran cooler too (can't verify that 100%, too many variables)

If you like to experiment you could play around with the width of the ports. It actually helps but be careful.
I widened the transfer ports (there are 2 in a 3 port engine) and the boost port (the one opposite the exhaust) by close to 1mm each. Make sure you are not raising the timings, just make it wider and keep all angles the same as they were. It's pretty tedious. You need to firmly secure the liner in something and use a good cutting tool for the Dremel. A magnifying glass helps too.
I felt this mod improved overall power. It was noticeable in the low to mid range. The track wasn't long enough to see how much it did on top.
Like I said, be very careful. If the timings are changed it can end up being bad and the engine won't run right.

Widening the exhaust port also works. It will give a bit more top without sacrifising bottom end. Raising the exhaust timing by 5-6 degrees will also give more power on top but you will loose some torque. By increasing exhaust timing you are effectively moving the power band up in the rpm range. The engine breaths better at high rpms the higher the port is but the torque down low is decreased.

A 'low power' engine like the OS RG has a very moderate exhaust timing of 160 - 162 degrees which makes it easy to tune and not super sensitive to weather conditions. It also has a lot of torque but will not have as much power at high rpms.
A top end engine like the MAC.21 (for example) has an exhaust timing of around 180 degrees. It makes gobs of power up top but it's weaker down low.
Both the MAC and OR RG are 3 port engines (as is the OS V-Spec) which is a 'classic' design. More ports is not equal to more power like some people say. The benefit of more ports is that you can actually use a bigger area of the cylinder wall that way.

What I mean by that is this:
You can always make a port wider but only up to a point. If they become too wide there is a risk of the piston actually tipping on its side a bit and get stuck. Instead of making one wide thransfer port you can then make two and have some support for the piston. At the same time the are of the two can be made bigger than the single port. This is (one) of the reasons some engines like the C5BB has 3 exhaust ports.

Phew, I'm tired of writing. I need a smoke