Gabi, with regard to the 5 hp mower, if you post the same information as you did previously for the 8 hp mower (the long list of numbers stamped on the pressed metal cooling air cowl around the outside of the flywheel, just below the starter cord) I'll be able to tell you when the engine was made. If it is still the original engine, the mower will have been made about 6-12 months after the engine. FYI, the engine in your 8 hp mower was made on 15 September 1986.
It sounds as if the problem your husband had in starting the 8 hp mower was in the safety circuit that keeps the engine from starting unless the clutch is disengaged and there is someone sitting on the driver's seat. All the safety circuit does is ground the kill-wire (that is, the engine-stop wire that stops the engine when you turn it off). This happens to many people - they want to run the mower for diagnostic purposes, or they are hand-starting it rather than using the electric starter, and so no one is sitting on the seat when they try to start it. The pressure switch on the driver's seat grounds the kill wire whenever there is no one sitting there, because it's safer not to have the machine start without a driver. Most likely when he was checking things out after it wouldn't start, he has pulled the wire off the switch under the driver's seat. As soon as he did that, it would start normally.
Most parts will be available for the Briggs and Stratton engines in both mowers. There are universal technical overhaul manuals for all the Briggs and Stratton engines in the manuals section of this site (but you have to subscribe to that area to get access to them). Minor overhauls, including replacing the piston rings, are fairly routine operations, and are likely to be well worth doing if the 5 hp engine is using oil and/or blowing blue smoke in its exhaust.
Transmission parts for those old machines may not be available new. It seems to be usual for people to get second-hand parts from a similar machine that is being dismantled for parts. There will be several people here on Outdoorking who may be able to supply them, when you know exactly what you want. Alternatively you might put in an entire second-hand transmission. The old Colts seem to be very common, though they are not all the same: "Colt 5" could mean any of several fairly different machines. If you can post pictures of the Colt 5 mower, one of our members can probably identify it, and we can try to find out what transmission it has. That would be a step toward figuring out what is wrong with it.
