yep, i built one last year using a newer schwinn stingray frame and a chrome springer frontend. check the lower frame bar on my bike, it's straighter than the lower bar on the old style stingray frames.
mine is like a lowrider, but it doesn't sit as low as most lowriders do...but it's the same concept.
My friend did build a lowrider, and it's so low that the pedals will scrape the ground...you can not peddle his bike because it sits so low.
one problem that you may have is getting the engine to fit in the frame...it's a tight fit.
i got mine in and it took awhile to figure out the best way to mount it (angles) so that everything fit. get the engine with the angle plug head or you will not be able to get the spark plug wire on and off, or the spark plug out once the engine is mounted in the frame. i have a hard time getting my spark plug wire on and off, and it's very tight getting the spark plug out....and i have the angle plug head.
you will have to modify the stock exhaust pipe or it may hit the ground, or get an expansion chamber (an expansion chamber is highly reccomended, because it adds a lot of power) you will have to cut the rear fender for chain clearance.
does that bike have regular laced spokes? (like 36-38 spokes) if it does, you should have no problem mounting the rear sprocket to the rim. some of those lowrider rims have a ton of spokes, (like 68 or something) and they can be straight laced. if it has those kind of rims, the spokes will be in the way of the sprocket mounting bolts.
my bike has straight laced spokes, but i think there are only 48 spokes (i never counted then) and it was difficult for me to get my sprocket bolted to the rim.
another thing to consider is that most engine kits come with either a 44 tooth ar 41 tooth rear sprocket. these sprockets were designed for 26" wheels, so you will not get as much top speed out of it as you would if you had 26" wheels.
I have a 41 tooth rear sprocket on my bike, and my engine is a 50 c.c.
i have added an expansion chamber, modified stock muffler, high flow air filter, accell spark plug wire and an ngk spark plug.
my bike will top out around 25 mph, but if i had a smaller rear sprocket, it would go faster. the trade off by going to a smaller rear sprocket is that you will loose some bottom end torque. but since a 41-44 tooth sprocket was meant to go on a 26" wheel, going to a 38-26 tooth sprocket should not hurt the bottom end torque too much with the 20" wheels. if you are seriously going to build a low rider, be prepared to run into problems and you will have to modify stuff to make it work...but it can work.
the other thing...look close at my engine. my engine does not have an intake manifold like the 80 c.c. engines do. my carb is bolted right to the cylinder. the 80 c.c. engines have an intake manifold that is about 2 inches long. the intake manifold puts the carb closer to the seat post tube, so you may have clearance issues there. but there is a place that makes an intake manifold that puts the carb out to the side of the engine (a 90 degree intake) so there is always a way to work around problems.
just don't get the idea that it will take you one day to assemble it, and be riding it right away. it will take time to figure everything out, but it isn't that difficult to do.
here's my bike...i built it last summer. i recently replaced both of the whitewalls with blackwalls. both the rear whitewall and my new blackwall is a cheater slick.
i also recently added a shock sissy bar, like on the old schwinn crate bikes. my seat sits as low as it can, and with the shocks on the sissy bar, it rides like a dream.
