try looking for the information part by part. Aftermarket manufacturers put up plenty of info so that engine re-builders can be sure they are getting the right part. Pistons for instance, try
www.jepistons.com will give you straight away:
Bore, stroke, rod length, comp. dist (squish), Head cc's, piston bowl/dome cc's.
in some cases you may get valve diameter and angle although try a cylinder head specialist for this if not.
The less high profile and secretive the more likely these companies will supply you with a basic but functional engineering drawing to check against your existing design (assuming you had one). Try spoofing a couple of companies, i.e. I am building a 2005 Mazda 2.0l, can I check your drawing to see if the pistons will work. If they start asking spec's bluff them and say it'll be turbo'd with a long throw crank, shorter rods and a deck plate with high lift cams or some such nonsense. Say you want to save money and fit shelf parts rather than going down the custom route, please send a drawing etc.
Con-Rod maufactures have not much to hide, length, B/E and small end dia and width and maybe weight will all be on their websites.
Cylinder block could be tough to get details of inside the casting unless you have the casting machining drawing (these are about, even for 15 year old engines) but you will get the basics from measuring one or looking on tuner wesites for decking measurements (top to bottom face) and this will give you crank centreline position for many engines with old stlye main caps. I see the drawing above has already provided some good info.
Manifolds should be straight-forward, there are many exploded drawings of webber DCOE carbs circulating (scrapyard for injection throttlebody otherwise?)
Crank will be a pain in the butt maybe, hard to find info, even harder to measure one, maybe someone can help there?
That leaves...Pump? Sump would vary depending on engine layout, as long as it fits...
Decide on a common engine, one with a cult or heavily tuning oreinted following is good i.e. Toyota 2.0l from corrola's and celica's, its called 4g63 or somthing, Mitsubishi lancer/evo engine has a similar name - 4...something.
Scooby flat 4's, ford duratec or the old cosworth iron block, VW iron block etc. look at the owners clubs to find links to tuners/suppliers and history of the engine.
If you told me which engine your particularly keen on I might be able to dig up some stuff. I've had to do a lot of reasearch in the past but it has since been scattered to the four winds. Don't wait for it though, I could be a while or simply give up!