Hop up your RM125 X Z D

First off put some Boyesen dual stage (Power reeds) in it
You can get them from your local dealer out of there Parts Unlimited offroad book Part #614 retail is $23.95
Shy away from porting. I remember back in the old days that the hot set-up was raising the exhaust port for more top-end but engines did not last too long after that.
Any easier top end hop up is try and locate a PSI or CRF cone pipe for that bike. Those Pipes gave those RMs huge top-end gains but remember to richen your main-jet if you do.
Stick with the stock 12/51 gearing unless you are having a hard time coming out of corners then bump to a 52 rear sprocket. (All Suzuki rear sprockets are the same from 1980 - 2007) so finding rear sprockets is easy.

Put 1981 RM80 steel clutch plates in to replace the 125 Aluminum ones. Your clutch action will be perfect with that swap

81-86 RM125 and 250 shocks all interchange

Drill three 1" holes in each side of the airbox (Cover inside with foam) That airbox was great for keeping out water but kept air out also.

One more thing
In 1984 Suzuki went with 43mm forks (1981-1986 triple clamps interchange) Get some 43mm forks along with an 84 - 86 front wheel set-up.
I have 86 clamps on mine with 1987 CR125 forks/wheel and disc brake. (If you run AHRMA then discs are not allowed) 2006-10-19