I had the same issue on a 'Made in China' frame I've recently found and restored. Initially tried to source Monty integrated seat bolt clamp ... no longer produced, out of stock. Solution: the Mini Velo seat with a quill seat post Tartybikes has on sale.
Info on the bike: RunWin Trials (2005)
Geometry:
Wheelbase: 1045mm; Chainstay: 384mm; Headtube angle: 71 degrees; BB height +20mm; Fork: 410mm; Stock Handlebar width (Flat bar profile): 28 inches; Stem: 80mm, 40degree rise; Cranks: 175mm (22T)Top tube: 559; Head tube length: 110mm; Seat tube length: 8 inches
Replaced the V-brakes to Maguras, changed: bar/stem, BB, tires, convert the front V-brake to Shimano/SRAM Centreline disc brakes, Shimano rear derailleur w/ 7-speed freewheel (pending single cog conversion with tensioner), paint KTM Orange (RAL-2009)