I have heard everyone on here talking about the head gaskets leaking oil . Most of the vehicles that I see 103 or 104 end up having more oil leaking from the upper and lower timing covers than the head area. We have done tons of head gaskets and timing cover reseals on these motors and most important is to replace all head bolts with new and reseal the timing cover with the proper sealant that wont let it leak again . I have one car that I did the head gasket and timing cover on and that was 130k miles ago and it has not leaked a drop yet. Several of our customers that come in for routine service that we have done this job on are still high and dry. I think the secret behind getting the gaskets and timing covers not to leak is a well cleaned surface before re-assembly and the proper sealant on the timing cover. I did my 103 motor 3 years ago and it is dry as a bone to this date ( 115k miles)
As for seals and guides well I have only had 1 104 motor with that problem and it was due to the vehicle had been overheated before and the pistons were egg shaped. But like time all things go bad ( seals).