there is no difference in pre 2000 splendors and afters.
The only difference is that pre 2000 splendors came without secondary air injection as a result they are less strict on emission control. one more difference was that the old model came with 45 teeth rear sprocket as a result any one riding a old model will say that they had better pull but the difference is that the old model had 45 teeth rear sprocket and after 2000 model has 43 teeth rear sprocket. so after 2k model has better top speed than old model.
Before 2004 model all had Japanese spares but it is not a matter as basically you have to change new spares into Indian one and believe me the Indian ones also last real long.
This is from a owner of 2001 Hero Honda Joy which has ridden for 2.8 lacs and still going strong like the day i had bought it from the showroom.

Oh really !!? You mean I can refit mine with the older 45 teeth rear sprocket and get greater pull (torque) with a straight fit ?? That would be great ! I need better torque on mine (2010 model , 6k kms done yet) , don't really need to ride it beyond 60 ever . Is this part available with HeroMoto workshop and no other changes needed (chain & front sprocket) ?

yes you can get old model chain sprocket but it will come as a kit.
we have both new and old model splendor and i know both the bikes inside out as i do most of the repair on the old splendor, splendor+ and joy in my house as i have all the tools. Once even i have completely overhauled my joy engine by my own.
Yes it is a direct fit. all hero honda 100 cc bikes come with 14 teeth front sprocket but the rear teeth are different.
old splendor/old cd100/old ss/sleek- 45 teeth
splendor/splendor+/old cd dawn/- 43 teeth
Hero Honda joy-42 teeth
street- 36 teeth
passion/passion plus- 44 teeth
Passion pro/passion plus(bigger rear drum)- 44 teeth
splendor nxg/cd deluxe/ new cd dawn- 44 teeth(smaller drum with narrow chain and sprocket)
super splendor/glamor - 43 teeth(narrow chain and bigger drum0

All the chain sprockets are interchangeable except the bigger dia drum's sprocket cannot be fitted to smaller and vice versa.
In hero honda you can interchange any parts you wish from other models.