I cant help you with the cost of Rebuilding your Engine, it depends on the spec you want and how many parts you need or want replacing/upgrading.
Paint pricing depends on how many parts you need painting, most Paint Shops are reluctant to paint a complete set of lambretta parts, theres a lot of prep work to do...basically..the more prep, the larger the bill at the end.
Id recommend getting the bodywork painted and everything under the panels, Tank, Toolbox etc and forks, wheels, rims, Frame Powder Coated.
This should leave you with just the Leggies, mudguard, horncover, sidepanels, rear light housing and headset top and bottom to get painted.
Most Body repair shops will be willing to do your bodywork for about 500 £, (Panels are easier to prep than complicated shapes like hubs etc.
Its worth driving around a bit, take the body parts with you and look out for clean business premises with good equipment. Agree on a time scale as sprayers are renowned for not meeting deadlines
For good quality "Triple Plate" Chrome you should expect to pay between 80-10 £ for the sidecasing and the same again for the exhaust and U-Bend.
Both prices depending on the condition of the parts.
If they are badly pitted or have deep scratches, it will take more time to prep for plating and may involve more copper plating coats and polishing which will push the price up.
If the exhaust U-Bend is badly pitted, it would be cheaper to replace it.
Heres a guide to what the Powder Coating should cost ; (Including bead blasting)
Prices are based on Ral colours, (Double coating, powder base coat and powder top coat).
Metallic colours normally cost Aprox. 3-5 £ more, as does Clear coating.(Double coating, powder base coat, powder top coat, Powder clear coat)
Effect colours like flake and candys are usually more expensive, most powder companys wont have it on stock and will need to buy it in for you.
Wheel rims,Pair 10£
Forks, 30£
Front hub, both sides, 25£
Rear hub. 12£
Rear mudguard 12£
Tank 18£
Toolbox 18£
Air Filter box 10£
Air filter elbow 5£
Seat base 30£
Frame 100£
Engine casing, Incl. side casing and Mag housing 120£
The next step would be to polish the ally parts which you can do yourself and zinc plate small parts like brake cable levers, gear swivels, starplate etc. Re-zincing is cheaper than buying replacement parts so is worth doing.
The same applys here, shop around and get at least two quotes in. Try not to use industrial coaters as youll recieve an Industrial product afterwards. Industrial coaters are cheap...your parts will be fed into the normal process and you wont have a large choice of colours. If you get your stuff coated by someone whos used to doing Scooter parts and small batches you will get better results. They will also know what to mask off, theres nothing worse than stripping powder coating from a mag housing thats been completley coated..
Hope that helps
