Did you maybe hit a kill switch? The timing chain could have broke or skipped a tooth (usually only skips a tooth when is is stretched out. Does it even have a timing chain? I would think it would have timing gears).
Yeah those 450R s can be a pain in the @$$ to start Warm or hot. mine was like that and it was stock internals WTF? My buddy who bought it has the same prob and every one else with a 450r has the same prob even on the e start
you need 4 things to make any gas engine run. fuel, spark, compression, and correct timing. you say you have the first 2 now its time to check the other 2. timing may not be adjustable on your machine and might be preset. I agree with the 2 previous posts, check your compression - you may have some valve train issues.