Restoring motorcycles is never a cheap thing to do. Overall, its far better to buy a complete, running and restored (or mostly restored)item. Trust me on this. Put the money you have to start into a jar, and add whatever you think you might be spending on parts each month into the same jar (get a large jar). In a few months you may have enough to buy a going concern that you can ride around and improve as finances allow.
However, you will lose out on the joy that most of us get from taking a jumble of parts and slowly turning it into a running motorcycle, built just the way we want. This joy costs money which I have never been reluctant to spend. So, the best thing you can do is forget about the cost - its the price you pay for that special feeling that only us fanatics will ever experience when we find that elusive part, solve that insurmountable problem and go beetling down the road safe in the knowledge that if anything falls off, its our fault...
... And all the special friends you meet along the way.
Join your local BSA, Classic Vintage motorcycle clubs and invest some time by going to meetings and events.
There are two distance markets, the inter club market and the evilbay market.
Usually you will find several members with more stuff in their sheds than they know what to do with.
None of them want any to go to the "traders' and most would like them to go to some one who will appreciate them and possibly be around them when it is finished.
No one with a shed full of unfinished bikes trolls through the classified activily looking for some one to sell their surplus bikes to.
OTOH very few will give you the time of day if you rock up to a meeting and announce that you want one of them to sell you an M20 on the cheap. However ask for help in finding one or for evaluating one you have found and things become different.
However when you make yourself know to them & they feel comfortable with you then usually you will get offers
