I weighed about 165 pounds last time I dared weigh myself naked, and my seat is rock solid on its frame tube which is also solid. When I smashed the rack by dropping the bike and took it off, I didn't replace the lowest bolt and the seat arrangement is still rock solid. I will work out something with a shorter bolt but that's the current situation. I suggest that you look at why that seat tube is not secure. Maybe it is wrongly assembled. I hope it isn't just the wrong size for the frame tube.

Strida. com sell new ball joint kits quite cheaply. You can probably take a chance on ordering one and fitting it yourself. I would chance the price and order one in the hope that my copy will have the same dimensions. As for swapping out - success depends on whether the dimensional error is in the frame tube or the plastic seat bracket, if indeed the problem is dimensional error rather than simple misassembly.

The balljoint in the UK costs £9 I think. The seat moulding is even cheaper. I don't know where you are located, but there will be a distribution channel nearby for original parts. If the copy uses the same size frame tubes you will likely get away with replacements. .... All guesswork, and probably not a lot of help.

http://www.strida.nl/en/store/parts.php