What I did is use a Savage 51 tooth gear grafted onto a Kyosho gear with teh teeth shaved down to fit insid the larger gear. It actually works great and isn't too difficult.
But a real adapter is the proper way to do it for sure.
Jammin CRT MM/Neu 1515 1700kv
Losi 8IGHT MM/Neu 1512 1900kv
Kyosho 777
T4 MM 5700
B4 LRP
XX4 MM 7700
old losi xxcr, MM4600 4s lipo 70mph+