Leung from Hong Kong sent me these really historic pics, along with some information.
He found an old Kyosho R/C products catalog, which is dated 1980.7.15.

Bike 1: "Elec Rider". 1/8 EP motor bike. The figure is dressed in fabric clothing and is hollow to conceal the R/C components (possibly the receiver and/or receiver batteries so far I can remember). The is no side crush bar so the players need to pick up the bike if it falls.

Bike 2: "Harley Davidson" 1/8 in scale.

Bike 3: "Yamaha YZ250" should be 1/6 in scale. The plastic figure was sold separately. Unlike other R/C bike, this one could run on dirt ground.

Bike 4: Similar to Bike 3 but fuel powered by 0.1cu3 engine.
By the way, I think "THIS STORE FROM HONG KONG HAVE SOME HOP UP PART FOR THE MOTORCYCLE. EMAIL HIM AT jj0725@hotmail.com referred to this company:
There are also full front fork parts.

However I consider the parts are overpriced.
For example, the rim costs US$13 each and front fork costs US$32 per set.
A full set of such parts costs over US$150 but the R/C kit itself is selling only at US$55 here in Hong Kong!
Also as an ex-motocyclist, I believe the tuned aluminium parts will add weight to the bike and affect overall balance and performance.

Update November 19, 2003: While browsing into ebay.com I found these very interesting pics of an old YZ and another bike. Enjoy!