I've recently taken my mint MTH PS1 SF AA F3/7 and gutted it. I replaced the electronics with parts from two MIB RK PS3's. I'm not a MTH Tech, but learned a lot along the way. Fun experience, and worth it to me, as long as you assume your time/labor is free. If you think your time is worth more than $10/hr, Marty's probably right - may not be worth it unless you have done hundreds of these and know exactly what to do and have spare parts available. Even simple things like not having taps for micro - metric threads and assortments of wrong screw lengths/sizes can slow/stop you.
Things I learned - the RK PS3, while the same PS3 electronics board, doesn't have all the 40pin PS3 connector sockets populated in the header. You might need to learn how to move pins or build wires to get features you want. I wanted MARS light, but no pin in that socket. Didn't need rear headlight, so "borrowed" the socket/wire from that pin and had to move it to the MARS light pin. Then you need to find a new chain file for engine that activates the MARS light. Do-able, but takes time. Not necessarily as plug-and-play as a kit, but cheaper.
SF PS1 didn't have smoke unit, but it was possible with some fabrication, to adapt the RK smoke unit accessories to the interior of the F3/7, and get it to smoke. Not as nice as factory installed, but does now smoke.
I'll disagree on the kit price comment above. $200 or $175, or whatever the current PS3/2 kit price is, was in my case, much more expensive than the MIB RK PS3 donor diesels I bought. Plus I have the leftover trucks, shells, couplers, motors, and other parts that can be sold to recoup most, if not all, of the price of the donor diesels. The PS3 board can become essentially "free" if you are willing to invest the time. But IMO that's Marty's point - if your time is $$$, use it wisely.