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I have a 91 Mazda 323 with 250k on it, burning oil and leaking all other fluids. Thinking about dropping a motorcycle engine in it. Curb weight 2300lbs, no a.c. or power steering. This seem like a good build?

Ahh, a bland car, pretty heavy, needing many other repairs.
I would say no.
For that weight you need power.

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If I must be a one-man PC free zone, so be it!

When I think of motorcycle engine into car transplants, I think of Diablo Smart cars - the ultimate sleeper! https://www.youtube.com/watch?v=f8h9mFMyEQc

'Course there was a swap many years ago that was also memorable - Jaguar 4.2 liter V-12, mounted transversely, in the front of an original Mini...and street legal!

Unfortunately, a 323 is a bit heavy for a motorcycle engine IMHO. The Smart is 500 lbs lighter, even with its original diesel engine in it - with a motorcycle engine, it would likely lose another 100 lbs or more. A classic Mini only weighs about 1400 lbs, with its cast-iron A-series engine. Fitted with, say, a Hyabusa, a Mini would be down into Locost weigh range, with ease. Consider 1300 lbs or so, with 194 hp on tap...

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Scratch building, at continental-drift speed, a custom McSoreley-design framed, dual-Weber 45DCOE carburated, Zetec-engined, ridiculously fast money pit.

http://zetec7.webs.com/

Almost all the issues with this car are engine/transmission related, currently being driven 40 miles a day. Mine is the base model so no power steering, or A.C. to gave to work about. Understand It would need to
Loose some weight, stock motor and trans comes in at 350lbs+ (cast block) dry weight

O and engine spec only 80hp 90ftlb when new, so not coming from much

If it's a hatchback, you might try something like Chuck Beck's Shogun! https://grassrootsmotorsports.com/articles/sho-time-original-festiva-shogun/

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Mike - Read my story at http://twinlakesseven.blogspot.com/

You should not go backwards on the power to weight ratio.

Think Hayabusa in a Smart Car!

Motorcycle motors also don't care for heavy chassis as the clutch basket is not heavy enough for the job.

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Honey anyone?

Looking at specs alone would seem like even a 1000cc bike engine would increase my power, most are avout 150hp and 75 ft lb torque? And yes it a hatchback

When I think of small hatchback engine transplants I think of rear-mounted jobs, like the Renault 5 Turbo...or Jay Leno’s Ford Fiesta. Serious fun, there! There’s video of Leno’s build online - the 323 might be a perfect starting point!

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Scratch building, at continental-drift speed, a custom McSoreley-design framed, dual-Weber 45DCOE carburated, Zetec-engined, ridiculously fast money pit.

http://zetec7.webs.com/

The spirit of the cars mentioned above were what Chuck Beck tried to capture with the Shogun! BTW, Leno has one of these too.

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Mike - Read my story at http://twinlakesseven.blogspot.com/

OK, just figured what's going on. Leno's Ford "Fiesta" turns out to actually be a Ford Festiva... That's the Shogun!!!

https://www.youtube.com/watch?v=3XR9SoQU16c

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Mike - Read my story at http://twinlakesseven.blogspot.com/

Looking at the shotgun, my plan was to drop a motorcycle engine in, cbr1000rr or larger, possibly a turbo, thinking maybe to a live rear axle. Then weight reduction to as light as possible?

i have done many bike engine swaps and i agree with most of the posting . a 2300 lb car is a little over weight for your average bike motor.
i would not do it with a sport bikes motor turbo or not.
you will lose some of the weight by removing the motor and trans as they are heavy. but the best cars to do bike engine swaps into are lighter cars like kit cars ,mg midgets, mini's, cars that scale in with there stock motors at about 1500 to 1700 lbs.