Go to motorcycle autojumbles and see what's there. It is possible to actually colect all the parts in one day to build a complete bike and by a workshop manual as well.
This actually works out cheaper than buying a complete basket case in one go. You do have a chance to pick which parts you buy rather than ending up paying for some useless, worn out parts, that you'll end up having to fork out more dosh to replace.
This is the way I do it and also my mates do it.
It is essential to have a workshop manual when you are restoring a bike. It gives you an idea of what parts you have, what you don't have, what are non-standard on your project and all the magic numbers such as valve clearances, points timing and gap, ring gap and other measurements.
Also the wiring diagrams, etc.
You'll also have a parts list and exploded diagrams, which are essential when hunting parts and fittings.
I've bulit whole bikes from various parts collected in this way.
It is sometimes the smallest little part that can hold up a restoration project. A friend is stuck with an RD250LC becasue he is missing an essential mounting bracket.
He's been trawling the internet and autojumbles for months so far to no avail.
My latest project in waiting is early 1970's Aprillia licence built Zundapp Sports 50cc of which I have just the sealed beam headlamp. I'll aquire the rest of the bike gradually and then start the build.
The more work and labour you put into a project yourself, the cheaper it becomes. Projects only cost money when you're have to farm out work to 3rd parties.
When rebuilding an engine always remember that the engine will only be as good as the quality of the bearings put into it, so fork out the extra dosh for SKF stainless steel racing grade bearings instead of general purpose cheap stuff. You'll be rewarded with a low friction, low MPG, improved performance, long lasting smooth engine. It'll also be longer before it'll need rebuilding again.
