I've used machined M8 bolts/nuts to replace Brooks nosebolts. If you choose well, you can access the hexagon in a machined allen bolt head and this makes adjusting the saddle tension much easier.
However most brooks models don't use a metric thread in the nosebolt so the nut often needs to be changed too, and the nut won't fit properly unless it is machined (eg as per photo in post above; with this arrangement the threads on the bolt are soon worn/bruised and this can inhibit further adjustment). However if you get the machining right (with a small taper on the reduced diameter portion of the nut) the nut doesn't need to be held when adjusting the tension.
You are probably thinking "I can't machine stuff". Well, if you have (or can borrow) an electric drill and an angle grinder, you can machine stuff like this; just hold the bolt in the drill chuck, set it (clamped down) to spin at some highish speed, and bring the angle grinder to bear. You will soon find that you can remove material quickly and easily, whilst keeping the part nicely circular still. Use a thinner disc for making smaller cuts and radii. Polish/deburr as necessary.
Additionally you don't need to machine anything if you don't want to; as mentioned in a post above Brooks Ti parts can be made to fit other saddles (which may require enlarging the hole in the nosepiece) additionally there are also parts from other leather saddles (eg the ones sold by spa cycles)
https://www.spacycles.co.uk/m2b0s204p0/Parts-and-Accessories/Saddles-Leatherwhich can be made to fit too.
BTW IIRC the spa nosebolts may be easier to fit than Brooks ones; ISTR the nut can be taken closer to the head of the bolt, thus making the thing shorter and easier to fit to an unstretched saddle.
cheers