You can do it!
I bought an XYZ Bikes Stretch Chopper couple of years ago, it is nearly the same size as the Schwinn XL-Adult size Stingray Chopper bottom tube measures 23-inches long. This XYZ XL-Chopper is about a foot longer than the stock 20" Schwinn Stingray Chopper!
My XYZ XL-Chopper does not require cutting out the seat down tube to fit in the chinese clone Horizontal Honda crf50 or sometimes called Monkey Bike engine, as does the stock 20" Stingray Chopper which has a bottom tube measurement of about 14-1/4" length, the Honda crf50 type engine fits in the stock stingray but very tightly.
Checkout the pic's below of the stock 20" Stingray Chopper & the XYZ Bikes version of the Adult Stretch XL-Chopper.
Stock 20" Schwinn Stingray Chopper, with seat down tube removed to fit the Honda crf50 type engine.
Here's the XYZ Bikes Adult size XL-Chopper, with the Honda crf50 type engine. Removing the seat tube is not necessary with this bike.
Yes you can stretch one of your stock 20" Schwinn Stingray Chopper's just buy yourself some steel tubing and start your project. This is a project that will cause hair loss & migraines for those who haven't the skills to take on this project!!!
I still say it can be done, remember if you have the patience !!!
C.H.
P.S. Did I mention that these 20" Schwinn Stingray Choppers sell for as little as $40-50 US-Dollars on Craigslist sometimes less than that if in poor shape, and the best ones that are near to mint condition sell on Craigslist for $50-100 US-Dollars. If they are selling for more than a one-hundred $100 and are in mint condition, I always haggle the price down to at least $75.00. So buy several of these Stingrays and you can modify them to what ever length you desire, maybe build a sidecar out of one using a less than perfect condition frame!