The 401 idlers I remember (we sold them fleetingly until 1975 or so when the 401's were quietly retired from production - they were awful rumble boxes by this time as we didn't have the dedicated setup and plinths available now and could be bought for £36 at discount) were hard compound I recall and nothing like the softer idlers as fitted to the 'lower' models including the Lab 80's, let alone the finely radiused ones as fitted to Duals, which I still think are amongst the finest idler decks ever made and with surprisingly good tonearms too. Maybe its the not used storage over decades but can't comment further on how the rubbery compound would react. I've never seen this myself on a Garrard, but again, I've never seen a terminally slipping 'Autoslim' idler that can't be dressed either, so maybe climate?
Rubber breaking down isn't a new thing. I had a section of a rubber drive belt (CD player loading belt) which was trapped between the clear plastic bags components come in. In ten years it had turned almost to liquid and it's a beggar to remove from carpets
One thing I'd say. The new-old 301 MUST have the motor bearings soaked in suitable lubricant before use and MUCH CONSIDERATION should be made of the main bearing spindle and how that is lubricated as sintered steel bushings need regular oiling apparently if the shaft isn't to get running marks quickly and if it's a grease bearing, the grease may be a tacky glue now - I'd recommend light lube on the thrust assembly as both 401s disliked flooding with oil as they rotated in sympathy if over oiled all over. These decks were designed for years of 24/7 running but please, only run the new idler in for 48 hours once fettled and not before.
They're silly-money expensive for well heeled audiophiles, but Loricraft is your friend here as at least they have the background knowledge and documents to make precise recommendations. If anyone out there wants to modify the main bearing, I'd say DON'T, but perhaps if the itch is there and the money is burning a hole in one's pocket, a total replacement could be considered - why, I don't know as it's the drive which causes the most noise, but I digress...