You can make them to suit different engines, i used to do mounts for the Pinto, zetec, R1, Fireblade and zx9 so there were choices to be had. As for brackets for the rad etc these really should be on the chassis but most kits don't have them... And some have bodywork that doesn't fit but heyp thats kit cars for you....

The beauty of doing it yourself at home is you make it to suit what you have but a company has to draw a line somewhere or they all become "one offs" and unless folk pay "one off" money it won't be cost effective.

Same applies to grp, the moulds make what the moulds make and if anyone wants something different then a plug needs to be made and then a mould, this all takes time and money.