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

Also put an 84-86 rear shock on your bike. 81-83 were junk plus in 1984 rebound damping became standard on the RM125s (1983 had compession damping but no rebound) Note---- The 84-86 rear shocks bolt right in with no modifying

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

1985 through 2004 CR125s use the same water pump seal.
1986 through 2004 CR80/85s use the same seal as the 125
1997 through 2004 CR250s "ditto"
1985 through 2000 CR500s "ditto"
Both CRF250/450s "ditto"

1976 through 1981 RM125s use the same coil and the 1979 and 80/81 RM100 uses the RM125 coil too.

How to tell them apart.
They share the same plastic (the front fenders are a little different) but the 1984 had a black frame and no powervalve and the 1985 had a silver frame witha powervalved motor.

-note
They both won the 125 shoot-outs in the mags and had the strongest motors.