My sister brought her 2002 Ka to me with a really bad missfire a few weeks ago, its got the older tappet Endura engine in it

I took a look and found number 3 plug wasn't doing anything, and traced it back to water ingress into the coil where the lead goes in. She'd driven it like this for a few days

I replaced the coil, all 4 leads and the plugs and all seemed well

Then she brought it back over last night with it having no power at all. On her drive over the exhaust also split at the flexi, so it was in a poor state! I took it to work expecting it to take me hours to get there, but i found that if i just feathered the throttle all was fine, it drove ok. If i 'booted' it though, it stuttered and coughed and really struggled.

I found the leads weren't on the coil properly (its really difficult to gain access), and when messing i managed to break one. So, new leads purchased again, fitted correctly this time (im sure!) and a full new exhaust and its still missing on full throttle?!

Im beginning to think maybe its burnt out a valve with it being driven with a missfire, although it was fine after the new coil and leads for a couple of weeks? Also, Is the exhaust blowing apart just pure coincedence? And finally, does anyone think that its cheap leads/coil thats causing it? I dont want to go and buy another decent set only to find they're the same?!

Ive checked all hoses, connections, and even reset the tappets, but still the same!

Help!!