Yz 125 slow going into powerband

SOURCE: I have a 95 yz 125 head and 98 yz 125 bottom end.

You have a 50/50 chance that they will work okay. All the mounting holes need to line up and the cooling holes as well. Get a head gasket for the '98 and see if all lines up. Will the exhaust pipe fit the '95?

SOURCE: should a 2 stroke tick over?

you just need to adjust your carb mixture screws located on the side of the carb this will adjust your tickover revs.hope this helps

SOURCE: 2005 YZ 250 bike has no power in the powerband motor starts

HI MATE ON THE CARB THERE 2 SCREW ONE FOR IDLE AND ONE FOR MIXTURE THE MIXTURE SCREW WILL BE THE SMALLEST UNSCREW A 1/4 OF TURNS ALSO TRY CHANGING THE PLUG OK

SOURCE: how do i decode yamaha yz 125 engin id number

You don't decode it. It is only a production run series number. Mororcycles are like cars in that respect. It is the frame number, AKA the "VIN number", is what carries all the information. Call a dealer with the frame number and he can tell you year, paint color, specific model and more. They may have a problem if the bike is too old. Yamaha changed to a newer code system a few years back.
Can I get a “very helpful” rating on this answer?

SOURCE: yz 125 1990 clutch spring diagram

www.bikebandit.com
It's one of the few that has a diagram.
Not sure what you mean by anti-rattle? The springs are for the pressure plate, and allow the clutch to grab, so you can go.