A 4.5 is silly fast and near completely useless in a 4wd buggy, especially a Lazer RTR. Even if you're limiting it via the transmitter it won't be helping your driving any. You'd probaby be a lot more comfortable driving something like a 7.5.
Your best bet would be to keep your eyes out for a decent second-hand Lazer and break it up for parts. Would probably be a fair bit cheaper than upgrading your RTR Lazer.
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