NON-RUNNERS
If the project you are considering has no practical chance of being put on the road prior to restoration, it might still be worthwhile investing a weekend in trying to get it up and running. This is, of course, assuming that the engine and its ancillaries are reasonably complete.
The first job will be to see if the engine is free to turn. So long as the brakes are not seized on, you can do this easily enough by putting the car in gear, opening the bonnet and rocking it backwards and forwards – if the engine is turning, you will see the drive belt at the front moving. If the engine appears to be seized, then try removing the spark plugs and running some oil into each cylinder against the bore walls (oil that sits in the piston crown does nothing to help!). Leave this for a few days and try again – if the engine is still seized, then it is probably not worth investing too much more time at this stage as you will invariably be looking at a rebore and new pistons anyway. As such, whether or not it currently has any compression is largely moot.
Assuming that the engine is free to turn, check the oil on the dipstick. If this is black and sludgy, then you don't really want to circulate that through the narrow oilways. Similarly, if it is a light coffee colour, then water has got into the oil and again you don't want to circulate this. If, however, the oil looks relatively clean and up to the mark, you are clear to go for the next two stages.
The first of these is to get it running. This requires essentially three things: compression, fuel and a spark. The ignition system might be good to go on a complete engine, though you may need to clean spark plugs and points. Do remember though that many cars get taken off the road initially as the result of a mechanical failure, so just because there is a distributor on the engine, you can't automatically assume that it is working, or even turning!
On the fuel side, many projects will have either no fuel in the tank, or stale fuel that the ignition will struggle to ignite. For the purposes of a static test, you are probably best to rig up a temporary fuel supply, either with a pipe leading from a spare can of fresh fuel to the inlet side of the fuel pump, or even a container of fresh fuel gravity fed into the carburettor. Just be sensible and work in a well-ventilated area, and do not create sparks where fuel vapours may be present – and that includes connecting and disconnecting a battery.
Despite all these precautions, it may not be possible to get the engine running. Carburettors may be blocked by residue left from evaporated fuel, rubber diaphragms may be split or any number of other issues stop it from firing. It is then probably only worth investing considerably more time in getting it to run if you have hopes of being able to put the engine back into service without a rebuild. In such cases, even if you fail to get it running just yet, it would be of real benefit to run a compression check by spinning the engine over on the starter motor – a gauge for this costs under £20 and will tell you the amount of compression, and if the figures are balanced across all cylinders. If, on the other hand, an engine rebuild forms part of your plans, then you will be able to measure and assess things like bearings and bores when the engine is dismantled.