The T50 should be able to cope with 180 to 200bhp at the flywheel. My conversion knowledge (no doubt someone will have other ideas)says that there will be conservative estimate of 30% drivetrain power loss so;
200 x 30% = 60
140 hpatw converted to kwatw = 104.4kwatw
It's not so simple either to just drop a W55 or other W series Gbox in. You will need to find a cressida 1 piece driveshaft and shorten it by about an inch and also stuff around with gbox mounts.
If you so want to make more horsies later then it's definitely no other option than to go the W series gbox though.