Thank dicko for the info of rcgroups and Novarossi.
I'm expecting Novarossi's moto engines to come out soon.

And thank Alan for the info of "backplate conversions".

That　is a very problem I have to solve,
and I have been in difficulties because I have no tools
to make such a roto start backplate to fit with new engines.

I found an article about Thunder tiger's original backplate
in 2WheelHobbies.com, and written there,
"Fits Pro .12 and .18 and Evo .12."

So,　for the time being,
TT's Evo-12 or Pro-18 engine is very likely to me.
but perhaps I have to wait Novarossi engines
if they'll come out soon and fitted with tiny roto start backplate.