Yeah...I had that same problem with the GRX28 motor on my inferno GT. I ended up getting a smaller flywheel. I'm sure kyosho has an optional part that can fit it.

Here is the smaller flywheel that should allow you to use a thinner washer or none at all, behind the flywheel. This flywheel may not working if your engine is not a pull start, because the flywheel will not stick out enough to engage the bumpbox wheel. If your engine is a pullstart, then everything should be fine. You should actually get better rpms with motor too.

Let us know how that turns out.