My favourite subject...Budget bikes.
Pre'65 can be as expensive or cheap as you want it to be. The size of your wallet and the boundaries of your imagination being the guiding forces. I know I use the example of my old mate Jonesy a bit much but he's such a contradiction when it comes to his philosophy on building bikes, using his experience is a good starter.
At the top of the pile you could build a shiny and rare ESO unit construction Metisse like Als famous Black Betty using the best parts money can buy and you wouldn't have much change left from 20k, that's if you could find one of the rare engines. His other big buck item is a genuine 1959 WORKS GP Monark 500 he's having built in Sweden that is costing who knows how much.
At the other end of the scale are his Sprite framed projects using $100 eBay frames and components.
While Metisse, Cheney, Mead, Puissant, Wasp, Hindall and other pre 65 legit aftermarket frames are big dollars no matter where you find them, the Frank Hipkin built Sprite frame has slipped through to the keeper with not many people picking up on the fact that they're pre 65 legal. The frames were produced from around 1964 to around 1973 and marketed as Sprite in the UK, American Eagle in the USA, BVM in Belgium and Alron here in Australia. They came with a variety of engines including Triumph,Villiers/Greeves, Maico, Husky, OSSA, Kawasaki 250 and their own Husky copy engine. Most of the engine applications place them in pre'70 or pre 75 but with a bit of thought a pre 65 bike could be created using a Sprite/American Eagle roller like Alans shown below. They're quality items built from Reynolds 531 tubing and many are nickel plated.
Below is Jonesys American Eagle (Sprite) frame as bought on eBay for $300 odd. 
Engines: As you can see, the Sprite frame has a rather large engine cradle that will accept a number of unit construction motors. The most logical as they actually used the motor in production is the Villiers engine. Engines from Greeves and AJS Stormers come up on eBay for around $1000 for a runner fairly often. If you want to stick to a 2 stroke, an early 360 Husky engine might be fun or, you can do what I'm planning and use a Maico 250 square barrel engine fitted with earlier alloy oval barrels.I was planning to use a 360 engine but I've been told the 360 oval didn't come out until 1965...damn it!
The Sprite frame will easily take a unit 500 Triumph T100 motor or BSA B44 engines or, if a tight budget is guiding your build plans, what about a BSA B44 350 engine? I'm not so certain that bigger unit twins like 650 Truimph or BSA A65 will fit the Sprite but a tape measure will answer that question when you're ready.
Forks/Wheels It's desirable to try and get a roller like above but if you can only find a frame, there are plenty of suspension choices. The bikes come with quality Ceriani copy REH forks and similar units can be often found for $100 or less. Betor or ridgey didge Ceriani forks are also easy to find for varying prices, the Betors being the cheaper and easier to find alternative as they came on a large number of makes and models for a number of years. CZ forks are also legal and are very good but are becoming more expensive in recent time. CZ magnesium hubs are also desirable but once again, they're becoming more expensive as supply thins out. REH conical hubs come up fairly regularly as do the almost identical Rickman 5 1/2" hubs from Rickman Zundapps or Rickman Montys. I've built up a stash of these hubs hubs and usually pay around $50 each on eBay.
As is the rule when building any special, get to know the class rules you're building for and study intently what can and can't be used. Taste is important so try and keep the bike looking pre 65. The standard Sprite tank and seat combo look the part but usable fibreglass tanks are getting harder to find. Aftermarket alloy tanks from Wassel, Lyta/HiPoint and other manufacturers often come up but the prices have spiralled in recent times into the 500 buck range for some dent free items.
The Sprite is only one of the cheap entry level pre 65 options. There are more like the Bultaco M11 Metisse and stock frames Brit machinery that we can look into if anyone wants to go further into this subject. Like I've writen on another thread, the pre 65 class has been avoided by many who are interested because of the percieved high costs but with a bit of lateral thinking a low budget but reasonably competitive machine can be built.
bELOW, THE 100 buck American Eagle Kawasaki frame that'll be the basis for my pre 65 Sprite Maico.
There's a good example of a pre 65 Sprite Villiers in issue 36 of VMX, page 32/33.
