I did similiar engine swap, but I have a farm spotlight it’s gone dim for some reason with the new motor would rectifier fix that?, don’t wanna switch to led but would rather stick with this light. It’s a Tecumseh 10hp

The only purpose of the rectifier is to change AC into DC. If you're running an incandescent light on it then AC will work just fine and no need to go through the trouble and expense of adding a rectifier.

If you light worked properly in the past I'd check the wiring to see if something is loose or corroded as a first step.

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If your new engine is just similar, it might have a different flywheel and alternator than your previous engine. There are different alternators (and sometimes corresponding flywheels), with different outputs. Maybe the last engine had a higher-wattage alternator than the new one does.

If this is the case, you might be able to swap the flywheel and alternator from the old engine, to the new one.

For instance, my blower has an incandescent light, and hand warmers. If this basic engine was used on a blower with just a light, they might have used a smaller alternator, since it would be powering a smaller load.

Good point. I hadn't thought about it but the OP should check the light itself to see what it's rated for. I always think it's going to be a 18 watt but it could be a 35 or so watt tractor light and the old engine had more output and ran it without problems. The new engine might only have the juice to run a single 18 watt light like most older snowblower engines or new ones that are set up to do headlight and hand warmer.

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Il have to check wiring again looked ok, original was 8hp Tecumseh, new ones the same but 10hp

For clarity, I wasn't trying to suggest that you wired anything wrong. Just that the new engine might have a weaker alternator than the old one did.