Hi All,
I got an inexperienced mechanic to fit new plugs which I bought but apparently they were the wrong size and the mechanic failed to realize my mistake and he went ahead and forced 2 plugs in to the plug hole before he realized they were 21mm plugs instead of 16mm. So he put back my two old plugs but left in the two big ones thinking they fitted well and not knowing they were stuck. After a 300km trip for xmas and back to the city I was driving from work on the motor way one of the 21mm plugs blew the coil cap (plug cover) off the engine. The summary is: I need to get two plugs out of my top cylinder , one of which is broken and the other is intact but since its too big there is no space for a plug spanner to fit around the plug to take it out. After 3 days of trying , the mechanic I use has told me my only option is to take off my top cylinder to get the plugs out, and I have been really avoiding this. Please what are the dangers of damage to my engine if the top cylinder is opened and put back, e.g timing chain settings ?
Note: I am in Nigeria and most our mechanics here just use basic tools, no computerized diagnostics and they usually are not trained in any way, so I want to be equipped which fore knowledge of the procedure so I can correct him if he is going wrong. Also the default attitude of mechanics here is to cause more damage to enable you extend your repairs and rip you off.

The mechanic has already collected money for a new top gasket from me , meanwhile another mechanic tells me I can still re-use my old gasket since it was not burnt and my engine was fine except for the wrong plugs. I will post more updates as I am scheduled to start repairs in the next 3 hours, that would be 9am here. Thanks in advance for responses.