0.25" Dia x 20 TPI is probably one of the following, but they are NOT interchangeable:
1/4" Whitworth (BSW - 55 degree pitch and a British Standard) or more lilely;
1/4" Unified Coarse (UNC - 60 degree pitch and an American standard);
With regards the length, a hacksaw is a great adjustment tool but make sure to deburr the end.
Please make sure you get a good grade bolt as your life does rather depend on it in this case!
As a general rule of thumb Allen/socket screws are a higher grade steel than your average bolt, so if in doubt fit a socket screw if you can't find the real McCoy but be careful about Stainless steel fasteners in cast iron/steel parts, they may mot mix as well as anticipated!
Please remember these are safety items - and that my rule of thumb in no way makes me liable for anyone's longevity!