The N21-3T is a fun engine for those who are realy starting this hobby and in Europe is is a cheap disposeable engine for racing in the rain (yes, we race in the rain).
As a backup engine you can use it but for an avarage racer it is to slow and a farce if you havre to use it at a higher level racing. But if you can get a crankshaft from an older faster motor like a 321/421/RX21, NovaMega EVO4/5/6, JP R01/R03 or MAX the N21-T3 will come to life.
There are faster Basic types as the N21-T5 and the N21-T7, and if you can get an hand on it the JP P7L is a big bang for a low price.