A stock N/A Cruiser can handle roughly 200hp (via light boosting turbo or 50hp shot of NOS)... after that you need to start looking into rods, pistons, valve springs, cams, pretty much an entire top and bottom engine rebuild. Once you get that done, you are going to have to do a new clutch and trans rebuild to handle the power. Bob Stockum and MX-5 are probably two of the biggest and baddest N/A engine builders. They've built N/A engines up to hold massive power, they'd be the two to talk to about 300HP+ engines.
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