Cheapest and quickest
solution would be go to the car accessories shop or a well stocked bike shop and get suitable rust treatment chemicals, some will just paint over the surface neutralising and stopping further corrosion and can also be painted over in a desired colour, some will strip the rust off AND the plating to bare metal and may also eat away at any alloy and some other metals, so make sure you get suitable ones! and protective gear, gloves etc. read the labels before buying!
If going down the route of using stainless and alloy fasteners, they MUST be of a suitable grade of material, inner casings like crankcases need higher tensile bolts, outer casings without loadings are less critical, but even there that may depend on what else is bolted to a scooters casings, some of it forms part of the chassis in effect. Also you may require sutable greases to fit the bolts, stainless is notorious for having rougher surface on threads, causing more difficulty in achieving correct torque settings, titanium bolts also require specific greases as well. There is also galvanic action or electrolytic corrosion to consider between dis-similar materials, another reason for the grease, this is accelerated if you are riding in winter with the salty roads.
Finally, do not use aluminium alloy bolts on safety related items like brake caliper and brake lever mounts, use steel, stainless steel, or titanium of high tensile specification.
Tabasco, are non of your bolts sent for plating, galvanising, sheradising, anodising, or any of the other processes suitable for different materials? how long do they last? are they being regularly re-covered in grease and oil? or are they in constant immersion in non corrosive fluids? or are we talking huge nuts (lol) and bolts on bridges with repeated paint or other coatings?