For a motor I would go with a 4mil and ported jugs, If ur going to be running 1/4 and 3/8 tracks. I run a stock car track (2 car tracks ) and my setup rocks. On the 3/8mil I was beating bigbore trx450 and yfz450 and one dude had a warrior with a 600 cbr motor in. For 1/2mil tracks a cub would be nice. I have talked to about 4 builder about a 421 cub for myself. they said it would only be good for the longer track. I would not have the bottom end you need to get out of the trun. So this year I still saying with my setup I got with a few more mods to run this new track. 3/8mil clay with 32deg banking . I go out for test day May 10th . ill tell you had I do.
For ice racing I raced last year I race 3/8mil track with banking 30guys on the track it was fun with my little 4 mil I was out front and was lapping ppl. Some guy on a z400 stop in the middle of the track it was hint him and go for the snow bank. I went for the snow bank. bent my pipe hanger, riped the boot, and the wiring fell down on the pipe and f*cked it . But I had fun doing it .
Hope this help if u need help pm me and I can call you.