Okay let's keep this simple and work our way up.
-gas in the bike? Mixed correctly?
-gas valve on?
-key on? ( if it has one)
- kill switch in the on position? ( if it has one)
- may have a tether. Have that hooked up?
If it has none and just starts or the above scenarios check out;
- did you use the choke?
- going on its a 421 I'm gonna assume aftermarket carbs. If that's the case, did you use both choke knobs?
If all that checks out;
- did you try changing spark plugs?
- if not, do it. And for shits and giggles keep one out and kick. Check for spark.
- this bike is new to you. Where did you get it? Did you drive from your mountain top home down to the valley to pick it up?
Ask questions. Someone will be able to get it sorted out. But start with what I said and then we can work from there.