Hey vazzo, there are still plenty of pinion gears available on ebay. Here is a link to a store that has a few and they ship outside the U.S.
http://cgi.ebay.com/MUGT0714-Mugen-M...2em118Q2el1247
I still have the stock pinion gears that came with the kit and they are in pretty good shape. Ive had to replace a few spur gears but those are still easy to find here in the states.
As far as running the MTX4 gears Ive heard you can do it. Ive heard you can swap out the entire clutch kit from an MTX4 and install it to whatever engine your running on your MTX3. Your obviously going to have get the correct spur gears to match the new pinion gear size. The gearing on the MTX4 is a bit different than the stock gearing on the MTX3. I recently installed a new engine in my MTX3 and I was thinking about doing that swap but I never went through with it. I figured if the clutch Im running now ever gives out I will go for it.
By the way, my MTX3 is running great with that new O.S. 12TZ! I just finished breaking it in last week at the track and I'm really looking forward to fine tuning it and running it all out in a few weeks.