Hi Morsilla,
First, MTK passed away a couple of years ago; so information on running gassers in pattern has been almost none existent. Second, I would run the Rossi. Much better power to weight and it will run with a larger prop and longer pipe. I would try a 15 x 8 or 15 x 9 prop (you can get a 15 x 9 from an Aeronaut prop distributor).
Good luck!

I have not owned or run a Rossi 90, but Rossi makes a great engine! Larger props running at lower rpms are a relatively recent trend for 2-stroke engines. The pattern fliers have been doing it for years because you get more vertical pull, a bit less horizontal speed which makes it easier to stay in the 'box' and less noise. Check the prop recommendations for the OS 'AX' series of engines; a lot more diameter than the 'good old days'. I think your Rossi will turn a larger prop, especially with a long pipe length. Also, using a larger volume pipe helps, too (90-1.20 size versus a '90' pipe). You can try a 14 x 8 and work your way up in size and increased pipe length. I will check the set-up I had on a K&B 1.00 turning a 14 x 10 APC with excellent performance (and no pump). I am running 15% nitro and you mentioned in the other thread that you live abroad and don't have access to nitro fuels which may impact the maximum prop size you can reliably run, but pipe length usually plays a more important role. The Rossi 90 in the Excelleron should provide plenty of power!