Lot's of information on this site already.
I find going to Google and use site:dt125r.co.uk 4bl derestriction better than the internal search.
https://dt125r.co.uk/topic/3102/derestrict-yamaha-dt-125-r-1997/66?page=4
It does depend on what region you live in, but the easiest way to get full power is
Zeeltronic Ignition
DEP Pipe
Make sure you've got 3MB cylinder head (3BN denotes you've got a combustion ring head that benefits from machining out). But otherwise you'll be laughing!
