Will a engine rust or become weak if the motor is left open for a long period of time?

It sure will.

Most of the time its the temp changes that causes moisture to form on the steel parts inside the engine, crank, bearing, rings , piston pins and some rods. This is why it is important to fog an engine for long storage.

but at the moment there is a whole bunch of coolant sitting in there will it still rust, and if so i guess i should button up the motor eh?
the motor needs work so ya,

Were is the coolant sitting? In the coolant ports? Crankcase? belly pan?

crank case i only have the cylinders off

Remove that prestone from there asap. And pour oil in it.

why will it hurt the motor?

Coolant is a mixture of ethylene glycol and water!!! Not designed to be in contact with high load hardened steel parts. If the crankcase has drain plugs take them out, if not mop the coolant up with some lint free rags and coat everything heavily in fogging oil. Spray it into everything you can in the crankcase and wipe down any of the removed parts. This will save you money in the long run.

you can also use a kerosene pump to siphon it out,,,, I had to put a smaller hose into the one that was on mine and then siphon it that way,,,, then hit it with fogging oil,,,, longer it sits, the more you are opening yourself up to problems

This is why you always drain down the coolant before removing heads and cylinders. The antifreeze will strip away all the oil from the bearing surfaces leaving them open to and prone to corrosion. Antifreeze on the inside of an engine is very bad and will ruin it. At this point I would suggest that you remove the motor from the sled and if you don't split the case at least turn it upside down and flush it out with WD-40. After you flush it out you can spray fogging oil on everything, or fill the case with injection oil if you want to leave it for extended periods.

Youll be fine, just suck that stuff out, when I did the same thing last summer (mine was apart for 2 months, waiting for new cylinders) I just sucked out what I could with a battery tester, a baster anything will work). As long as there is just a little left it will be fine. JUst pour oil all over in there (I alwasy use the same oil I run in it), WHen you are ready for assy (and even rigth now if you are worried) put a rag in there and spin the crank, then use an air gun. All will be fine!