Seems to me if you have been reasonably happy with 6 year old engines then you don't need the expense of an engine builder to get a big performance gain that you will be more than happy with. The last fraction of performance in these engines comes from very regular rebuilds, after which they plateau at a level all but the most competitive will be happy with.
However you do need to be reasonaby mechanically competent to do the work, noisy 6 year old engines will almost certainly need rings, valves and possibly camshafts. Unless you are really racing without ever checking tappet clearances then the tappet noise you can hear is probably worn camshaft journals, where they run in the block. You might also want to uprate the engines to UT1 spec at the same time, which means pistons as well.
Be careful because before you know it you could get close to the cost of new engines. If it was me I'd buy a pair of new G4's, (£210 ish each) transfer all your external bits to them and put what's left on ebay. If you're lucky you could see a net cost of only a little over £300 to end up with 2 new engines..
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