If you want reliability you should do a 4mil serval. I had a FAST 4mil serval for a cpl years myself and it was great in the trails and hill climbs in the drags it would run with 4mil dune port stock cyl just fine and my serval only had a clean up port. It had power ALL over the low end. A 421 cub is basically a drag ported serval and is geared more for drag racing. If you were to do a 421 serval you would get about 70-75hp out of it with a clean up port. I had mine with rocket pipes, +4 timing, cool head with 24cc domes, 35 pwks, vf3 reeds and upgraded clutch. It ran amazingly well, never had a problem, and I got about 800 hours on it. I spent aprox $2500 on all of that and never regretted it. Jeff at fast did an amazing job for me. Now i am just about done with a 10mil serval from redline but that is another story. But you will also want to consider front and rear suspension (a-arms, swing arm, and shocks) and upgrade all of that at the same time, it is well worth the $$$. After all of that is said and done you need to be ready to spend $3500-$4000 I would estimate. But the nice thing about it is that once you do all of that you are set up pretty decent if you ever want to do a 10mil build. Also I would do a dual pingle for sure and you may consider a pro mod tranny, shift shaft and star mods and possibly a lock up clutch. I did not do the tranny, shaft, star or lock up with mine but it is a good idea to do if you ever want to go bigger or port the serval any. Hope this helps man