Ok i have been wondering about that one too, and with the website you supplied i think i found a chip that would replace the TIL198 (apparently not made any more), it is KP1020E (RS# 395-6209). I base this on this on this
webpage. It states that a TIL198 is a dual optocoupler with darlington inputs which the KP1020E fits perfectly. As a side note wouldn't it be better to get K10100B (RS# 395-6344), since you are only using one optocoupler?
To use for the 555 chip i am choosing TLC555CD (RS# 788-334).
For the 4017 i am choosing CD4017BCN (RS# 345-5403).
hope this helps you, you sure helped me.
Update: well i guess if i lived in the UK this would work for me, oh well i spent enough time puling my hair out trying to find people who sell these chips in the US.