This thread just exploded in good advice with the previous three replies. Did you even try $250 or $275? However, $300 is chump change for me after making a paradigm shift in my own thinking...
We're in Bidenomics and inflation is pretty bad, I get it. If you're stuck in late-1980s thinking, you might still think
OMG $100 is a lot of money but it isn't. $100 is just a Benjamin, like $10. $200 for all that junk is a steal and $300 is just finally a respectable amount of money but nowhere near outrageous.
YMMV.
Baja Doodlebug -- Those are almost a current-era classic. They're not made anymore but the closest thing is a Coleman-something.
MTD - Classic minibike. Ugly and overbuilt for their time but they have their fans.
"Azusa" - Still manufactured and yet still a classic.
All 3 are rollers and not some rusted out yard art.
So 1/3 of the asking price is a fully assembled roller that you want (with a Tecumseh on it). You do the math:
https://azusaparts.com/product-category/kits/minibike-kits/