87' Indy 600 motor options and other build opinions?

I am currently in the process of building an old school muscle sled. The goal is to make it more powerful and more powder worthy without ruining the sleds trail manners too much and without ruining the overall retro-ness of it as I have a huge soft spot for pre-wedge Indys. I bought the sled with a 650 triple in it but it would have to be rebuilt including the crank. So that got me wondering if I should just look for an ultra or storm motor instead? I have never seen one of these motors in a pre-wedge chassis and was wondering if I would have fitment issues with pipes under the hood and such. I lack the skills and equipment for any serious fabrication so would definitely appreciated any input here. Not really looking to do any other engine mods other than pipes as I don't want reliability to take too big of a hit. Other upgrades I plan on doing are rail extensions with a 136in track and 1.25 or 1.5in lugs, xtra 10 suspension front and rear, plastic skis, and new or re-done seat. Will also look into lightweight parts if I can find them for a reasonable price. Any input in these areas would be appreciated as well. Thanks

87' Indy 600 motor options and other build opinions?