Luke,

The reason people choosing Mintor, OS Webra is the rear exhaust or wrap around header with pump.

3M 140 only weighs 768 gram while the Moki 135 weighs 820 grams.
That is about 2 oz. That is not bad at all compared to OS 140 or Webra 145.

Having fuel tank on CG, you will need pump or at least regulator. Probably a little more weight for pump or regulator (1 oz)

If using a tuned pipe to use on Pattern type planes, you will have to add tuned pipe to the Moki price.

That is $300 + header ($30 ~ 40) + Tuned pipe ($100) + pump ($30 for Perry, $60 for Cline regulator).

Total for Moki 135 in pattern $460.00 minimum. It is not bad at all. I've never tried Moki, but heard it is very good engine.
I have no idea how much the Moki 135 costs in US, so I assume it is not very much cheaper here.

The reason for 3M mintor is the lightest weight in its class. Those serious guys are willing to pay more for light weight engine and 3 oz weight saving.

I know there was at least one guy using Moki for FAI plane at World championship, but most used OS & YS. No idea why.