JXK makes a very good point about crankcase fretting, if you have oil leaks from the crankcase joint and the cylinder bases the chances are the crankcase has fretted.
To continue to run an engine if this is happening will write off the crankcases and so cost far more money than you save from continuing to run the engine.
The engine shop that Peter recommends above is one of the three in the UK I would recomend.......... And my personal choice of engine shop.