The kit you bought will do the job, its just one of the more expensive options.
New intake runners aren't available other than the ones that have those problems. There are various ways to soften old rubber parts, including soaking in various concoctions that include Oil of Wintergreen.
I've had surprisingly good results by coating them with silicone grease and letting them sit for a day before wiping off the excess. It won't make them soft right away but the greasiness will help them go back on and I found the ones I did that way much softer when I took them out again a year or 2 later (I suspect the heat cycles draw the silicone into the rubber).
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