Welcome to the site !!
It could be jetting being clogged , and/or valves needing adjustment or replaced(Intakes usually) , as for jetting for your elevation (a good place to start) , here is a guide as to where to start , if your bike is popping I would suspect that the Pilot is clogged OR it is the wrong size for your elevation , they are around $6.00 so if it is a #42 for example , then replace it with the correct size (#45) **you will need to look closely near the tip where the screwdriver slot is , but on the side , you will see a number like 42 or 45 , etc. , that is the jet size
This is a jetting guide to go by , follow it and you should be good
Jetting Guide for Main Jet:(pilot 45) CRF450R/X
Sea level....165
2000 ft.......162
4000 ft.......160
6000 ft.......158
8000 ft.......155
Temperature/Elevation:
One main jet size (up/down) for every 2,000 feet or 25 degrees in temp.
Check the Fuel screw , it is located at the front of the bottom of the bowl , you might have to tilt the carb base toward you to adjust it , unless someone has replaced it with a aftermarket .... set it somewhere in the area of 1-3/8 to 2-1/8 turns out from lightly seated (do not snug it down or you will damage the tip of the fuel screw)
You should invest in either a Owners Manual , or a Factory Honda Service Manual , they are filled full of all the info you will ever need to do maintenance on your bike , including rebuilds , and suspension maintenance ..... you can get them at your local Honda dealer , or buy them online , or buy one here ..... (the 07-08 Factory Service Manual is basically the same as the 05-06)
https://www.crfsonly.com/catalog/produc ... ts_id/3226
Also , some starting tips ..... When the bike is cold , Turn on the gas , pull the choke , and give the throttle 4-6 full twists , then put pressure on the kickstart and slowly rotate the engine till you feel it get on the most compression (run it thru a couple times , so you know what the highest feels like) then get it on the highest compression , then allow the kickstart to come all the way up , then put your weight into it (like starting a Harley) and give it a good firm full kick , the bike should start right up , or close to it ..... this is basically how you start a 450 , it needs RPM to get that mass going , so you have to be firm when kicking it , you can't kick it like a 2 stroke ......
If you are trail riding the bike mainly and not track riding , then I would look into either gearing it down , either temporarily or permanently and/or adding a heavier flywheel or a flywheel weight , it will make going slow so much more pleasant , the stock gearing would be around 13/48 , I would go with a 13/50 or up to 52 (you will need a new chain......what I do is buy a 120 link chain , then buy 1 or 2 extra master links for it , this will allow you to use the chain links you removed , so swap to different rear sprockets , so you can experiment with gearing and find what works best for you)
Also , look at the clutch basket , if it is worn it will have grooves , they will make the clutch grabby , and make it harder to take off smoothly .... this is what a worn basket looks like
As far as oil changes go , it really depends on how you ride ... I ride my 02 on the track every weekend , I change oil about every 6 to 10 rides (each ride being in the 2 hour range give or take , so roughly every 12 to 20 hours depending on how dusty it is , or how hard you push the engine , I look at the oil and see what color it is , if it looks clean , then I keep running it till I see it get darker/dirty (look in the oil level window on the ignition cover) .... if you are just trail riding , I can see 20 or more hours not being a issue ..... the "recommended" intervals are for Racing , and just a "recommendation" not a mandatory interval ... I personally use Full Synthetic , it lengthens the intervals , and to me , helps with carbon build up and smoother shifting .... Petroleum based oils , I would change a bit more frequently than what I suggested .... and yes there are 2 oil locations , the engine side and the tranny side .... the filler for the engine is on the ignition cover (the twist off cap) , the filler for the tranny is located on the clutch side (the twist off cap) , both drain from the same side , but different drain holes .... The engine drain is located at the bottom of the the ignition cover just at the tip of the shifter .... The tranny drain is located directly behind that in the recesses just forward of the front sprocket (basically look at the ignition cover drain bolt , and the tranny bolt is just behind it back in farther)
690cc for Engine side with filter change (filter is located where the 2 screws are in the oval cover at the front side of the ignition cover just above the oil window)..... I run a full qt. in the tranny side (it won't hurt anything , and help with shifting ...run what I recommended in the engine side)
Hope that gets you started on your journey .... if you have any more questions , feel free to ask ...that is what this place is for .....