SH makes the HPI engines, maybe you can check them for parts.
Otherwise any .12 or .15 engine from any manufacturer (OS, Novarossi, Picco, SH, Traxxas, etc) is a drop in solution.
I have a Picco .12 in my RS4 2 kit.
And is a MUCH stronger performer than the original motor.
But then just about anything would be.