The 88cc kit keeps the bike original looking and retains whatever electrical system that came on it (6 or 12v.).
My '80 "Frankenport" was originally purchased off of eBay for .99 without a front wheel, seat, head, and various other parts. After finding out how well the semi-auto 125cc Lifans ran after dropping one in my street-legal XR-50 pit bike and riding 450 miles in two days on an annual dual-sport run held in the Hoosier N. F. I bought another one for the Passport. You can't beat the price--$258 to your door or $299 with a carb and electrics. I made my own simplified wiring harness using the one that came with the motor kit and adding to it to wire up a head and tail light. I use a $15 voltage regulator from Tucker Rocky and an $8 halogen tractor spotlight from Harbor Freight as a headlight with no battery in the system.
The Lifan is wider than the Passport motor on the right side and requires either modifying the stock peg bracket or the purchase of a footpeg assembly from the guy selling the motors (under $30). The cylinder is longer on the Lifan than the Honda motor which in turns moves the exhaust forward around 1/4" or so. To make it run well you really need a pipe that flows better than the stock Passport exhaust. I'm using a CRF-70 high pipe with no spark arrestor screen. A vendor on eBay called classicscooterparts sells a head pipe that would be an excellent start for a free breathing low pipe--just add some sort of large volume muffler to it.
If you already own a 12v. Passport I'd go with the Lifan. If you own a 6v. unit and can live with that electrical setup then go with an 88cc kit. They make a huge difference and are as reliable as stock. I ran one for years in a CRF-70 motor in all kinds of use and abuse and it's still running as well as the day I put it together. For reference I used the Honda Trail Bikes 88cc kit which came with a carb but I never could get it to jet up right and ended up using a rejetted stock CRF-70 carb. These bolt right up to the stock Passport intake/air cleaner tube. Modifying the airbox to flow more air (drilling it full of holes?) would probably help a bunch.
On the Lifan you just run a pod style filter. Currently I'm running a K & N only because I got tired of the legs of my jeans getting oily from foam filters.
I'm running a throttle assembly from a BBR XR/CRF-50 handlebar kit and bars from some mini-moto bike on the Lifan/Passport with a home-made bar mount setup that bolts in where the stock Passport bars bolted in. I'm not sure what you could do to adapt this carb to the stock 70 setup--maybe a custom cable from Motion Pro?
If you went with an 88cc kit and a CRF-70 carb the stock Passport throttle cable, etc. MIGHT work.
The biggest gain the Lifan has over an 88cc modded Passport motor is the additional cog in the gearbox which makes a huge difference. Running a 14/36 combo my Lifan/Passport will hit sixty MPH yet still has plenty of zoom around town. Adding a four speed gearbox to the Passport can be done with an aftermarket kit but that's more $$$.
Hope this helps...