i dont know if they are vostok, but there are third party titanium forks and triangles sold on british ebay. they are expensive enough so that buying a second-hand ti brommie seems a better idea, especially if you need a spare bike, and who doesnt? you then get the pedal bolt, chainstays and sometimes seat post titanium as well. and lighter headset, lighter wheels with a different front hub and reduced spokes.

the derailer i use is a long-cage road dura-ace. it doesnt quite tension the chain, so a steve-parry tensioner could be handy. birdy uses one quite similar. longer cage mtb derailers might be better, but you have to see to that the chain does not get too close to the ground. with an mtb-derailer a gear changer made for straight handlebars is easier to find.