After doing those basics you may have to buy a rotary tool and use a cutting disc to raise the exhaust port some to get the extra speed you desire. Maybe 1mm at a time to see how much increase you get. Also lower the intake port the same amount. Both have limited port durations that limit speed no matter what else you do to the bike.
It's a tricky tradeoff though, as far as I understand, you get more high end power by increasing the timing, but lose out on the low end (which is likely why you said to increase intake length). Jenning's book says increasing exhaust port area (in any direction, up, down, side to side) will boost power, some high power motors use up to 70% bore width (and some even more but they used bridged ports), but a 62% bore exhaust port should be safe to do (about 30mm wide). you can get more power still by raising it, but there's that timing tradeoff, the morinis pull 9hp at 11.5k RPM IIRC ([torque x RPM]/5252). Using that equation you can see that more power can be had by simply increasing the max RPM but of course there's a limit to that thanks to physics (and size limitations, your ports can only get so tall haha) being that our motors dont exactly have strong materials, we need to boost power within our RPM band, which is relatively short (which is good for beginners because we can focus our efforts). Some claim 11,000 RPM, but I dont think that'd be safe, I'd agree 9000 RPM absolute maximum for safety. So essentially, choose your target max RPM, gear appropriately, balance the crank for your RPM, build a torque pipe, widen the ports (and raise them to match your target RPM). raise compression, raise case compression to ~1.5:1, correct the transfer port flow, put on an intake header, reeds if you choose, and a proper sized carb. Perfection isn't easy, but it sure is worth it.