You should be able to use the IRS cover and bush without any modification, as these are the replacement parts for that model of chassis. The inner bush, as you have stated, is the problem. I don't know how it compares with the outer IRS bush, as regards OD but it may be that it is simply compressed and the smooth outer rubber bush might just do the job.
The thing to remember, when comparing urethane with rubber, is that the two types work entirely different. The urethane ones are like regular bushes which must be greased so that all the parts slide against each other well. The rubber ones are compressed into place and must not slide against the spring plate or cover. These bushes (installed only with talc powder) twist internally, which is also why they last so long and are not affect much by dirt etc. So, perhaps you should stick with rubber, though genuine VW would be best.
The thing to remember, when comparing urethane with rubber, is that the two types work entirely different. The urethane ones are like regular bushes which must be greased so that all the parts slide against each other well. The rubber ones are compressed into place and must not slide against the spring plate or cover. These bushes (installed only with talc powder) twist internally, which is also why they last so long and are not affect much by dirt etc. So, perhaps you should stick with rubber, though genuine VW would be best.
