Well i would do wat I'm doin....take out the .15 or Pro.15......sent it to traxxas with like 130 and they send you back(if you choose it) a trx 2.5 or 3.3 with the conversion kit with engine mounts and an exhaust and the mounting hardware.....I'm gettin the 2.5r for mine because 3.3 will start to kill all my parts but i should be fine with 2.5r....plenty of power....you could that or also buy a new O.S. 18 for almost 300...but thats a lot for a nitro sport....i got nitro stampede and I'm gettin like i said the trx 2.5r.....JMO
I'm not a fan of Traxxas engines. There are other .18s that will work too, OS is the most popular. I'd look for a good buy on an .18. BTW, the 2.5R is still a .15 just ported a little better for a bit more power than the Pro .15.