Comet makes an economy 1" clutch that sells for around $60-$70 here in the states, but I've seen guys post that 1" clutches fit pretty loose on a 25mm. engine shaft. Seems like someone here posted that they had fixed this problem with some sort of shimming between a 1" clutch and they're 25mm. shaft but I couldnt find the thread.
If you look into using any 1" clutch model, something else to consider is if your engines keyway is the standard 1/4" size thats usualy used with 1" clutches.