I have not done it on a 420a but I have done it on 4g63s and on the Isuzu V6 on my SUV, neither one was really that hard, its just a lot of work taking it apart but if you have a manual or follow the vfaq directions its not that hard http://www.vfaq.com/mods/timingbelt-2G-NT.html From the vfaq directions it looks pretty similar to the 4g63, it might be easier because there is no balance shaft belt. If you are going to change the timing belt you should really change the hydraulic tensioner and idler pulleys at the same time, I didn't do that on the Isuzu and the tensioner went out about a week after I changed the timing belt, fortunately the Isuzu is not an interference engine so no damage was done I just had to do the whole timing belt job over again
